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Some new species of Mastigophora and Sporozoa and a Microfilaria, parasitic in fish, frogs and toads in eastern Canada, are described and illustrated.Trypanosoma percae n.var. canadensis from the yellow perch, Perca flavescens, is monomorphic, smaller, with a shorter, broader body and shorter free flagellum than Trypanosoma percae from European Perca fluviatilis. Trypanosoma myoxocephali n.sp. from the longhorn sculpin, Myoxocephalus octodecimspinosus, is small, with a narrow undulating membrane. It is the first trypanosome to be recorded from Myoxocephalus. Trypanosoma lavalia n.sp. from Bufo americanus is delicate, with a prominent undulating membrane with two myonemes. Trypanosoma gaumontis n.sp. from Bufo americanus is small, crook-like, with undulating membrane with deep folds and one myoneme very-near the bordering flagellum. There is no free flagellum. Trypanosoma montrealis n.sp. from Bufo americanus is long and sinuous, with very short free flagellum and without chromatoid granules in the cytoplasm.Trypanosoma rotatorium (Mayer) is described from Rana catesbiana from eight sites, R. clamitans from three sites, and R. pipiens from three sites in Quebec Province. Trypanosoma inopinatum Ed. & Et. Sergent is recorded from Rana catesbiana and R. pipiens, A form similar in dimensions and morphology except for a much shorter free flagellum has been found in R. catesbiana from St Eustache.Spirochaeta manitoui n.sp. is described from the blood of Rana catesbiana from Lake Manitou.Lankesterella canadensis n.sp. from Rana catesbiana from Montreal and Lake Manitou is described and illustrated. Intracorpuscular forms have polar vacuoles, schizogony occurs in endothelial cells of the blood vessels of the liver and kidney, and multinucleate oocysts have been observed.Haemogregarina myoxocephali n.sp. from the longhorn sculpin, Myoxocephalus octo-decimspinosus, and Haemogregarina urophysis n.sp. from the white hake, Urophysis tenuis, are described. Two other haemogregarines sens lat. from species of Rana and Bufoamericanus are described, Haemogregarina aeglifini Henry is recorded from a new host, Gadus callarias from off Labrador, Haemogregarina platessae Lebailly from Pseudopleuronectes americanus and Haemogregarina bigemina Laveran & Mesnil from the eel pout, Zoarces angularis, the hosts being new.Plasmodium catesbiana n.sp. from Rana catesbiana from the Back River, Montreal, is a small species, with ‘signet ring’ and amoeboid trophozoites, schizonts with up to eight merozoites clustered round coarse pigment granules and gametocytes showing slight sexual dimorphism. Plasmodium bufonis n.sp. occurs in Bufo americanus from Laval des Rapides. The schizonts are large, oval, and give rise to eight merozoites arranged en barillet, the pigment usually being at one pole. Possibly micro- and macromerozoites may be produced. Macrogametocytes are oval and deep-staining; microgametocytes are vermicular. Both have pigment.A haemosporidian, different from but probably allied to Haemogregarina bigemina, is described from the black sea bass, Centropristis striatus.Haemoproteus laurentiae n.sp. occurs in Bufo americanus from the banks of the St Lawrence at Montreal South and at Lake Manitou. Its pigment is characteristic, consisting of rod-like or bacilliform distributed granules. Haemoproteus lavalia n.sp. from Bufo americanus, from Laval des Rapides, has pigment restricted to curved bands of rounded to somewhat angular granules. Uninucleate parasites have been observed in extremely few endothelial cells of the lungs. Haemoproteus lanoraiea n.sp. from Bufo americanus, from Lanoraie, has very pale yellow to light brown pigment granules, densely crowded at one pole and forming a compact granular cap. Oval parasites have been found in endothelial cells of the lungs.Leucocytozoon salvelini n.sp., parasitic in the leucocytes of the speckled trout, Salve-linus fontinalis, is described.Dactylosoma salvelini n.sp., parasitic in Salvelinus fontinalis from a Laurentian stream, is the second species of Dactylosoma to be described and the third to be notified from fish. It has fan- to wedge-shaped schizonts producing eight merozoites. Its gametocytes are not markedly different from one another in shape but differ in cytoplasmic staining and in nuclear structure. Dactylosoma sylvatica n.sp., parasitic in Rana sylvatica from near Quebec City, produces eight merozoites and has oval and reniform gametocytes.The morphology of a Microfilaria from Rana sylvatica is described.The effects of the Protozoa on the host cells are described for each organism, and some differential leucocyte counts for comparable normal and parasitized Amphibia are given in the concluding section.

Summary. An illustrated account is given of Plasmodium relictum n. var. spheniscidæ from the King Penguin, Aptenodytes patagonica, from South Georgia, the Black‐footed Penguin, Spheniscus demersus, from Saldanha Bay, South Africa, the Yellow‐crowned Penguin, Eudyptes antipodes, from Foveaux Strait, New Zealand and the Rock‐hopper Penguin, Eudyptes crestatus, from Gough Island. The variations in the morphology and in the range of dimensions of the Plasmodium from each of the hosts are described and illustrated. Detailed comparison shows that the same organism is present hi all of them. The effects of Plasmodium relictum var. spheniscidæ on the hosts are described in some detail, counts of infected erythrocytes and of parasites are given and the effect of alterations induced in organs of one Black‐footed Penguin are discussed. The parasite level in the different hosts is noted. The identity of the Plasmodium from the four species of Penguin is discussed in some detail, especially in view of the wide distribution of Plasmodium relictum Grassi and Feletti both geographically and in the number and variety of its hosts. Various morphological and physiological strains of P. relictum are noted and induced morphological differences by passages to unusual hosts are considered. The conclusion is reached that a strain or morphological variety of P. relictum is present in Penguins, for which the name Plasmodium relictum n. var. spheniscidæ is proposed. The low gametocyte level of P. relictum var. spheniscidæ in all the hosts seems to indicate that it may be approaching natural agametogony, and so may be on the way to self‐extermination. Tolerance and age‐immunity of hosts and possible susceptibility and immunity among Culicine mosquitoes in relation to avian malaria are briefly noted. The economic importance of avian Plasmodium in general, and possibly of Plasmodium relictum var. spheniscidæ is briefly indicated.

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