Tilapia ovary cells (TO-2) cease to proliferate when moved from normal growth temperature of 31 degrees C to 37 degrees C, and arrest in G1 and G2 phases of the cell cycle. The ability of the arrested cells to re-enter the cell cycle when restored to 31 degrees C decreases inversely with time spent at 37 degrees C. A heat-resistant strain, TO-37c, cloned from the surviving fraction of TO-2 after heat treatment, has been found to re-enter the cell cycle with greater facility and to have a higher rate of survival. TO-37c cells have a smaller cell volume than TO-2 and show a distinct morphology at 37 degrees C. Most of the heat-shock proteins (hsps) induced on temperature change were similar, but in TO-37c the decline in the synthesis of a 27 X 10(3) Mr hsp was faster and a 37 degrees C-specific 60 X 10(3) Mr hsp was missing. Ultraviolet (u.v.) sensitivity was slightly affected if heat treatment was given after irradiation. However, when cells were preheated and then u.v. irradiated, the u.v. sensitivity increased sharply for TO-2 cells but not for TO-37c.