Tissue phospholipids ofCulex tarsalisCoquillett collected as adults or pupae from breeding waters located in Kern County, southern California, contained an essential nutrient, the fatty acid eicosapentaenoic acid, in proportions that were three times as high as for routine laboratory-reared adults. Proportions of triacylglycerol eicosapentaenoic acid, although always much lower than for the corresponding phospholipid fractions, were also much higher for wild than for laboratory-reared mosquitoes. Another essential fatty acid, arachidonic acid, was present in phospholipids in similar proportions for wild and laboratory-reared mosquitoes, although levels of triacylglycerol arachidonic acid were usually higher in wild than in laboratory-reared material. These mosquito-essential fatty acids were detected as substantial proportions of the total lipid fatty acids of sludge and detritus from the bottom of fourC. tarsalisbreeding waters, and both fatty acids were major components of the lipids of three types of microcrustaceans that characteristically abound in the mosquito-breeding waters. The ecological significance of these findings is discussed, and their possible bearing on the sexual noncompetitiveness of laboratory-reared adults is noted.