Growth of Aedes aegypti cultured cells was arrested by α- and β-ecdysone at concentrations of 0.01 to 10.0 μg/ml. The inhibitory effect was accompanied by increased cell volume. Prolonged exposure of at least 24 hr to the molting hormones was necessary to induce the above effects. 3H-α-ecdysone was incorporated into the mosquito cultured cells, and 2.1% of the total label added could be detected in thoroughly washed cells. Thin-layer chromatography of the cellular butanolic extract revealed one apolar peak only and no traces of the original labeled α-ecdysone or its immediate hydroxylation product, e.g. β-ecdysone. It is suggested that the hormone was rapidly converted to a metabolite which did not correspond with the apolar 3-α-dehydro-ecdysone, as was initially speculated.