The distribution and abundance of protozoa in two chalk streams were studied. Particular attention was given to the ciliates and amoebae associated with the dominant submerged macrophyte, Ranunculus penicillatus. Highest numbers of protozoa occurred in habitats in the stream where the current velocity was reduced. Mean standing crops of ciliates, flagellates and amoebae were 32.5, 2.9 and 2.0 mg dry wt m−2 of stream bed respectively. Estimates of production and respiration rates by field populations of ciliates and amoebae were made for one type of habitat within the streams. The results are compared with published data on the productivity of protozoa in other habitats and of other invertebrates within the chalk stream ecosystem. The limitations of such productivity estimates are discussed.

Laboratory growth rates of six species of Gymnamoebia, isolated from English chalk streams and cultured on bacteria, have been determined at four different temperatures. Generation times ranged from 4.46 to 33.3 h. A linear relationship between log specific growth rate and the reciprocal of the absolute temperature was demonstrated for four species. A significant regression of log generation time on log cell volume was obtained for data on amoebae in combination with data on ciliates taken from the literature. This regression may be used to predict the growth rates of other species of amoebae and ciliates of known cell volume.

The population densities of the main groups of protozoa in the principal protozoan habitats in a chalk stream were surveyed over period of a year or more. These data were used to estimate the mean biomass and annual production of flagellates, ciliates and amoebae in the different habitats, and to make comparisons with the estimated production of macrophytes, algae and bacteria in the stream. Protozoa epiphytic on macrophytes accounted for about 30% and protozoa in soft sediments about 45% of the overall protozoan production. Lesser amounts were attributed to protozoa in stony sediments, those epizoic on insects and crustaceans, and least to protozoa in the water column. Ciliates contributed about 75 % of the protozoan production, flagellates 15 % and amoebae 10 % . The total annual production by protozoa was estimated to be approximately 16 g dry wt m-' stream floor, which was equal to about 20% of the bacterial and about 1% of the algal production. However, protozoan production was of the same order of magnitude as that by Gammarus, and more than the production by fish.

The numerical abundance of peritrich ciliates epizoic on larvae of Brachycentrus subnubilus in an English chalk stream is described for an l&month period. Peaks in the epizoic population occurred during winter at a time when protozoan populations in other stream habitats were low. Ciliates were absent from the first three instars of their host, but were abundant on fourth and fifth instar larvae. Estimates of generation times in the field were made for the ciliate populations (January -February 186 h; July -August 471 h). The relative importance of the epizoic ciliate population as a whole is considered in relation to other protozoan populations in the streams.