Dieldrin-resistant strains of the yellow-fever mosquito, Aedes aegypti (L.), contained 20–50% more larval fat than their susceptible counterparts. The total lipid contents of various substrains of a given strain were closely correlated with their LC50 levels. The increase in fat involved both the neutral lipids and phospholipids. Since similar differences were observed between larvae within a given strain, it is considered unlikely that the mere increase in lipid content is the resistance mechanism.