Hydrocarbons comprise 54% and 76% of the cuticular lipids extracted from the wings of L. migratoria and S. gregaria respectively. The hydrocarbon fraction of L. migratoria contains 17 n-alkanes ranging from C 14 to C 33 in chain-length, while that of S. gregaria contains 11 n-alkanes with chain-lengths ranging from C 22 to C 33. n-Nonacosane is the major consitituent of both fractions. n-Hentriacontane and n-heptacosane are major components of the L. migratoria and S. gregaria fractions respectively. The major branched alkanes of L. migratoria comprise 3-methylalkanes and mixtures of 9 + 13 + 15 + 17- methylalkanes . A dimethylalkane is also provisionally identified. The major branched alkanes of S. gregaria are mixtures of alkanes possessing one, two and possibly three methyl side chains. A comparison of the cuticular hydrocarbons of L. migratoria, S. gregaria and four other Orthopetera suggests that the hydrocarbon pattern of cuticular lipids could be a useful, additional character in insect systematics. Periplaneta americana resembles S. gregaria in having a high proportion of hydrocarbons in its cuticular lipid and in possessing a cuticle highly impermeable to water. Surface film balance studies show that a film of cockroach lipid spread on water does not form a tightly packed monolayer of polar molecules and it is suggested that the impermeability of the film to water is due to the formation of a heterogeneous monolayer, comprising mainly hydrocarbons, in which the molecules are optimally packed. A heterogeneous monolayer at the cuticulin surface could account for the high impermeability of cockroach cuticle to water, although frequent disruption of its molecular packing by newly secreted lipid makes this unlikely, and it is suggested that the outer regions of the cuticulin layer contain lipid and participate in the waterproofing of cockroach cuticle.