Previous findings from our laboratory have shown that the optic nerves of goldfish acclimated to different temperatures differ considerably in their glycerophospholipid composition. This paper describes changes in the morphology of the nerve with different acclimation and fixation temperatures. Optic nerves of 5 and 25 degrees C acclimated fish were excised and fixed at the temperature of acclimation, or at the reverse temperature, and the morphology observed by electron microscopy. Under all temperature conditions considered there is a statistically significant linear relationship between the radius of the axon and the number of myelin lamellae. However, the temperature of acclimation and fixation both influence the regression coefficients for this relationship, the higher the acclimation temperature the lower the coefficient and the higher the fixation temperature the higher the coefficient. The periodicity of the myelin also alters with these temperatures, being greater in the 25 degrees C fish than in the 5 degrees C ones. Myelin sheath thickness is also significantly greater in the 25 degrees C fish. These results are discussed in relation to observed changes in glycerophospholipid composition and conduction velocities.