The N,N-dimethylcarbamates are much less toxic and much more volatile than the corresponding N-methylcarbamates. N-acylation of N-methylcarbamates is also generally accompanied by a loss in intrinsic toxicity to adult mosquitos by topical application, but the reduction in toxicity may be relatively small and in some N-acetyl derivatives may not occur at all. N-acylation is also accompanied by changes in physical properties such as lipid solubility and partition coefficient, which influence contact toxicity, and volatility, which affects residual activity. In the series of compounds examined, contact toxicity is maximal in the N-acetyl derivatives but their volatility is often so high that they lack the persistence required of residual insecticides. This does not apply, however, to the N-acetyl derivatives of OMS-597 and OMS-708 because the volatility of the parent compound is so low that some degree of persistence can be sacrificed to obtain some improvement in contact activity.OMS-1064 is the only N-chloroacetyl derivative tested so far that shows promise as a residual insecticide.