Two different psychrophilic types of the heterotrophic nanoflagellate Paraphysomonas imperforata were isolated from Newfoundland coastal waters and the Arctic Ocean. When fed bacteria without food limitation, both isolates were able to grow at temperatures from -1.8 to 20 degrees C, with maximum growth rates of 3.28 day at 15 degrees C and 2.28 day at 12.3 degrees C for the Newfoundland and the Arctic isolates, respectively. Ingestion rates increased with temperature from 14 to 62 bacteria flagellate h for the Newfoundland isolate and from 30 to 99 bacteria flagellate h for the Arctic isolate. While temperature did not affect cell yields (number of protozoa produced divided by number of bacteria consumed), it affected flagellate sizes. This differential effect of temperature on cell yield and cell size resulted in a changing gross growth efficiency (GGE) in terms of biovolume; colder temperatures favored higher GGEs. The comparison of Q(10) values for growth rates and ingestion rates between the isolates shows that the Arctic isolate is better adapted to extremely cold temperature than the Newfoundland isolate. At seawater-freezing temperature (-1.8 degrees C), the estimated maximum growth rates and maximum ingestion rates are 0.81 day and 30 bacteria flagellate h for the Arctic isolate and 0.54 day and 12 bacteria flagellate h for the Newfoundland isolate. Our findings about psychrophilic nanoflagellates fit the general characteristics of cold-water-dwelling organisms: reduced physiological rates and higher GGEs at lower temperatures. Because of the large and persistent differences between the isolates, we conclude that they are ecotypes adapted to specific environmental conditions.