Eggshell ultrastructure in 8 species of mosquitoes from the genera Aedes, Anopheles, Culex, and Toxorhynchites is compared using transmission electron microscopy. A 3-layered organization is present. The vitelline envelope is of similar character in all genera--homogeneous, rather electrondense, and with no substructure. The endochorion always includes a lamellate layer (with at least one lamella) and, except in Toxorhynchites, tubercles of varying size and shape. The exochorion is a thin layer covering the outside of the tubercles, sometimes like a sheet, but more often weblike. The ventral side of the Anopheles egg lacks exochorion entirely. The endo- and exochorion in Toxorhynchites are fused and contain numerous large empty spaces. Ultrastructural differences were found to be greater between different strains than between different species. It is suggested that species-specific characters should be chosen only after study of populations from all parts of species' ranges. Possible functional trends in the eggshells are discussed, as well as the importance of integrating ecological studies with morphology to understand how environmental and other factors act upon eggs. Egg characters suitable for phylogenetic analysis are suggested.