S U M M A R YThe fine structure of the spore sheaths of some Streptomyces species was studied by carbon replication of intact and freeze-etched specimens and by negative staining. Young vegetative hyphae lacked a sheath but developed one before the onset of sporing. Substructures were observed on all sheath material; these differed between species and in sporogenous hyphae at different stages of development. Four components were recognized : hollow or grooved elements; amorphous material ; fine fibrillar elements; and subunits of spines and hairs. The interrelationships of the components and the possible roles of the sheath were discussed. I N T R O D U C T I O NStreptomyces spore chains are enclosed during their development by a thin sheath. Spores of some species appear to be ornamented when examined by electron microscopy. The ornaments, which may be warts, spines, hairs (Kiister, 1955) (Wildermuth, 1972 a, b).When spores are detached from their chains, the sheath may remain partially covering them or they may become free from it (Williams & Sharples, 1970). Properties of spores, such as their hydrophobicity, are often assigned to the 'spore surface' and the contribution of the sheath is obscure. More information about the structure and composition of the sheath is therefore required. The results from a survey of sheath structure in several species are reported here. METHODS Cultures.The following strains were used : Streptomyces finZayi (hairy sheath), S. gZaucescens (hairy), S. griseus (smooth), S. thermoflavus (knobbly), S. venezuelae (smooth), S. viridochromogenes (spiny) and Streptomyces sp. (FI) (smooth). They were grown on oatmeal agar (Waksman, 1961) at 25 "C for 2 to 3 weeks, with the exception of S. thermofavus which was grown at 45 "C for 3 days. In studies of germinating spores, spore imprints were made on sterile coverslips which were then incubated in moist sterile sand for 24 to 48 h.

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An ecological study was made of the freshwater distribution of Actinomycete virus. One component of this, the virus of Actinoplanes, is highly specific and its pattern of occurrence mirrors that of its host. The occurrence of Streptomyces virus is more difficult to explain in ecological terms, but this is attempted. Micromonospora virus was not obtained and the significance of this is discussed.The Actinoplanes virus is characterized at the fine-structure level by a polyhedralshaped head with a tail bearing a contractile sheath, the latter feature being previously undescribed for actinophage.

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