The heat-shock response induced in Trichomonas vaginalis by exposure to various incubation temperatures was traced by metabolic labeling and monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Increasing the incubation temperature from 37°C to 43°C depressed normal protein production and enhanced synthesis of heat-shock proteins (hsp). Smaller increases in incubation temperature resulted in little change in protein synthesis, whereas larger temperature increases inhibited protein synthesis. The hsp produced by T. vaginalis included molecules with approximate molecular masses of 85, 78, 66, 61, 35–31, 20–15, and 12 kD. Trichomonas vaginalis switched to hsp synthesis gradually. Full conversion to hsp production occurred within 60–90 min after the initiation of the 43°C stress. The period of synthesis was different for individual hsp, suggesting independent regulation of hsp production. Four strains of freshly reinitiated and culture-adapted (extended in vitro culture) T. vaginalis synthesized the 85-, 78-, and 66-kD hsp, but varied in the synthesis of the 61-, 35-, 34-, 32-, and 31-kD molecules. Culture adaptation affected the heat-shock response of two of the four strains tested.