The molecular species composition of phosphatidylcholine (PC) was determined in plasma membranes of kidney for rainbow trout, Salmo gairdneri, undergoing thermal acclimation between 5 and 20 degrees C. Species of PC from 5 degrees C-acclimated trout were characterized by significantly higher values of unsaturation ratio (8.78 vs. 3.15), unsaturation index (2.32 vs. 1.84), and average chain length (34.18 vs. 33.12 carbons) than those of 20 degrees C-acclimated trout, primarily because of elevated proportions of 16:0/20:4 (8.67 vs. 4.42%) and 16:0/22:6 (15.61 vs. 7.18%) and reduced levels of 14:0/16:0 (3.34 vs. 12.41%). Proportions of saturated and monoenoic species responded most rapidly (within 16-48 h) to temperature change, whereas species containing long-chain, polyunsaturated fatty acids increased only after 10-21 days of cold acclimation. The combined results of the present and preceding paper [Hazel and Landrey, Am. J. Physiol. 255 (Regulatory Integrative Comp. Physiol. 24): R352-R357, 1988] indicate that the metabolic adaptation responsible for the thermal compensation of membrane structure differ in response times, which range from rapid adjustments in both headgroup and molecular species composition to considerably slower changes in the proportions of species containing long-chain polyunsaturated fatty acids. Furthermore, the restructuring of phospholipid molecular species composition modifies membrane structure in different ways at different times during the acclimation response.