AbstractMarine and fresh water fish were depleted of tissue unsaturated fatty acids to various degrees and subsequently presented with linoleic and linolenic acids at different dietary levels, at different temperatures, with and without other dietary fat.Examination of the tissue fatty acids demonstrated that marine and fresh water fish do not differ between themselves or from other classes of animals in the following basic mechanisms of deposition and interconversions of dietary fatty acids: The fish are readily depleted of tissue polyunsaturated fatty acids. Dietary linoleic and linolenic acids are deposited, the former to a greater degree than the latter. At high levels of linoleic or linolenic acids in the diet there is no significant degree of their conversion to the longer chain more highly unsaturated acids typical of marine oils. At low levels of linoleic or linolenic acids in the tissues there is a significant, but slight, conversion to the longer chain acids at low environmental temperatures. The increase in the level of linoleic acid in tissue lipids which accompanies increases in the dietary levels, quickly tapers off above dietary levels of 5%. Temperature differences between 13 and 23C had little or no influence on the deposition or interconversion of polyunsaturated acids. Dietary cottonseed oil, which contains cyclopropene fatty acids, produces an increase in tissue stearic acid in the fundulus.