1. The proportion of ethanolamine phosphoglycerides in microsomal fractions of goldfish intestine increases at low environmental temperatures. The fatty acyl composition also changes, the proportion of C22:6 and C20:4 fatty acids increasing in positions 1 and 2 and position 2 respectively. The proportion of C16:0 and C18:0 fatty acids falls in position 1 and there is an apparent switch of C18:1 and C20:1 fatty acids from position 2 to position 1. 2. The proportion of choline phosphoglycerides does not depend on the previous environmental temperature of the fish. Temperature-dependent changes in fatty acyl composition in positions 1 and 2 take place in a way similar to that described for ethanolamine phosphoglycerides, but in this case C22:6 substitution is confined to position 2. 3. Choline phosphoglycerides have been further separated into 7 different molecular species. The amounts of species 3 to 7 increase and the amount of species 2 decreases at low adaptation temperature. These changes only account for part of total change in fatty acyl composition. The remaining changes occur by chain substitution within species. 4. Present results show temperature adaptation to be highly complex, involving both quantitative and qualitative changes in different phospholipids. The possible physiological significance of these changes are discussed together with the effects these changes might have on cholesterol-phospholipid interactions.