1. 1. Crabs were acclimated for 3 weeks to cold and warm temperatures. Plasma membranes prepared from leg muscle were analysed for fluidity and lipid composition. 2. 2. Plasma membranes from cold-acclimated crabs were more fluid and had lower cholesterol to phospholipid ratios than those from warm-acclimated crabs. The ratio saturated/unsaturated fatty acids was little altered by temperature acclimation. 3. 3. Plasma membranes from the stenothermal species (Cancer pagurus) were slightly more fluid (at DPH probe level) and had lower cholesterol/phospholipid ratios than those from the eurythermal species (Carcinus maenas). 4. 4. Plasma membranes from crabs acclimated in spring had more saturated and less polyunsaturated fatty acids than those from crabs acclimated in autumn.