1. 1. Seasonal changes in carcass fatty acids occurred in free-living migratory white-crowned sparrows. Particularly noticeable was an increase in myristic acid and a decrease in linoleic acid at the time the birds were exhibiting premigratory fattening. This was probably due to a seasonal shift in diet from seeds to insects, particularly to hemipterans. 2. 2. Captives fed a constant diet did not show fluctuations in their fatty acids during the season. 3. 3. It seems likely that most seasonal changes in depot fatty acids are due to diet but there are indications that some fatty acids are preferentially stored or synthesized.