The effects of temperature on the growth rate and gross growth efficiency (GGE) of the heterotrophic nanoflagellate, Paraphysomonas imperforata, cultured from the Ross Sea, Antarctica were investigated using five experimental temperatures (range=0-20 degrees C). This bacterivorous protist exhibited measurable growth over the temperature range examined, although temperature exerted a significant effect on its growth rate. There was no evidence for an effect of temperature on GGE. The growth rates and GGE of our Antarctic P. imperforata isolate were compared to values reported for other cultures of species from this genus. A wide range of growth efficiencies have been reported for different strains of Paraphysomonas spp., but our estimates were comparable to mean/median values reported in the literature. The growth rates of our Antarctic P. imperforata were similar to rates obtained for an Arctic conspecific at low temperatures (0-5 degrees C), among the highest reported rates for any Paraphysomonas species at intermediate temperatures (10-15 degrees C) and similar to rates reported for temperate congeners and conspecifics at 20 degrees C. Q(10) values of 15, 2.2, 3.6 and 0.93 were calculated for growth rates at 5 degrees C intervals between 0 and 20 degrees C, respectively. Results indicated that our Antarctic P. imperforata grew at rates comparable to other polar isolates at ambient polar temperatures, but these low temperatures may be outside the physiological optimum for the isolate.