1. 1. The effects of temperature on the activities and kinetic characteristics of triosephosphate isomerase from the pyloric caeca of Echinaster sp. held at 25 and 15 C were measured. 2. 2. The apparent Michaelis constant (Km) of starfish collected in August (30 C) was significantly less (P < 0.05) at an assay temperature of 15 than at 25 C, indicating positive thermal modulation. The Km remained relatively constant from 25 to 30 C. 3. 3. The specific activity of TPI of starfish collected in November (25 C) was slightly lower (P < 0.1) in individuals exposed to 25 C than in those exposed to 15 C for 28 days. The Km values did not change with exposure of individuals to 25 and 15 C for 28 days. 4. 4. The activities and Km indicate that TPI undergoes no or only partial acclimation to temperature. © 1982.