Population growth rates of Antarctic and temperate isolates of the microflagellate Heteromita globosa Stein and of the ciliate Colpoda cucullus Müller were determined during incubation both under constant 3.5° and 8.0°C, and under temperature regimes fluctuating on 24-hour cycles, between 0° and 15°C, to simulate Antarctic microclimates. Fluctuating temperatures did not inhibit growth. No growth of Colpoda occurred at 3.5°C. It is suggested that successful growth in nature depends upon the microclimate providing sufficient degree-hours per day above encystment/excystment threshold temperature (+1.5°C for Heteromita about +4°C for Colpoda). The minimum number of degree-hours above threshold needed for growth of Colpoda is estimated to lie between 48 and 96. Monitored microclimates in the maritime Antarctic do not provide enough degree-hours per day, though subantarctic microclimates are more favourable, as may be exceptionally warm localities in the continental Antarctic where daily insolation is high in summer. These data are consistent with the recorded presence of Colpoda spp. in the subantarctic and rarely in the continental Antarctic, and their absence from the maritime Antarctic.