The activity of teÑuthrin, carbofuran, terbufos and dieldrin in soil against eggs of di †erent ages of the southern corn rootworm, Diabrotica undecimpunctata howardi (Barber) (Coleoptera : Chrysomelidae) was determined. Embryological development following treatment with these insecticides was also investigated to determine the stage of developmental arrest. Generally, younger eggs were found to be more susceptible, dieldrin being the least and teÑuthrin the most potent ovicide of combined egg age mortalities. Terbufos and carbofuran were relatively inactive against older eggs compared with teÑuthrin. Terbufos stopped embryonic development at the time when acetylcholine esterase activity has been shown to commence. Carbofuran, dieldrin and teÑuthrin slowed but did not arrest embryonic development, although the two former compounds prevented eclosion to a greater degree than teÑuthrin, suggesting death during absorption of serosal Ñuids at eclosion. Neonate emergence following treatment of eggs with teÑuthrin resulted in death within a few hours. The results are discussed in relation to insecticidal action and permeability changes of egg membrane structure with age.