Phagocytosis was studied in vitro using coelomic fluid of the Antarctic starfish Odontaster validus at 0°C. The number of coelomocytes present was determined and the phagocytic activity of the phagocytic amoebocytes (PA) was quantified with yeast during incubations of 1 and 2 h. The percentage of PA phagocytosing increased significantly from 42.29 ± 10.50% (SD) at 1 h to 52.57 ± 13.96% at 2 h. Numbers of yeast per PA also rose significantly from 2.27 to 2.45 cells per amoebocyte, indicating that phagocytic activity was maintained. In vitro phagocytosis of an Antarctic invertebrate at 0°C is shown for the first time, and the types of amoebocytes involved identified. Rates of phagocytosis were similar to, or higher than, reported data for temperate starfish, although this conclusion must be treated cautiously because of scarcity of data and differences in methods used. However, the data suggest that phagocytosis in O. validus is well adapted to low temperature.