Lipid hydrolysis and oxidation were evaluated by multivariate statistical methods in farmed rainbow trout (Oncorhynchus mykiss). The trout were acclimatized at 5 or 12°C and then frozen as fillets and stored at -15°C for up to 34 weeks. Results from analyses of lipid hydrolysis and lipid oxidation during frozen storage of light and dark muscle revealed that the major cause of lipid deterioration is hydrolysis. Hydrolysis resulted in an increase in free fatty acids by between 1 and 10%, particularly in the nutritionally important polyunsaturated n-3 fatty acids, 20:5 and 22:6. The phosphatidylcholine content of the muscle decreased by 25-50% during frozen storage. Phosphatidylethanolamine was stable in dark muscle but decreased in light muscle. By multivariate statistical evaluation it was possible to group the samples according to storage time or acclimatization temperature. A lower water temperature (5°C) resulted in reduced lipid stability during frozen storage. © 1995 American Chemical Society.