The German cockroach is convenient for assaying gonadotrophic effects of juvenile hormone (JH) analogues. The animals can be raised synchronously from egg to adult state in 2-5 months at 30°C, a shorter time than for any of the common laboratory-reared cockroaches. Last instar larvae can be stored for up to a month at 16 to 18°C and brought to 30°C to metamorphose when needed, providing a convenient source of bioassay animals. The assay takes 2 days to complete, less time than previously published gonadotrophic assays. The growth of terminal oöcytes of the ovary responds linearly with log dose between 0.08 and 0.8 μg of Cecropia JH-I. The analogues JH-I ethyl ester and methyl-juvenate have dose response curves parallel to that for JH-I, enabling potencies, r, relative to JH-I to be calculated (r = 1.01, 0.35 respectively). There is no significant effect of the volume of hormone vehicle between 2 and 8 μl. A 30 per cent increase in the precision of the bioassay is obtained by correcting the oöcyte growth for the fed-weight of the animal. The bioassay is adequate to resolve a twofold difference in potency using as few as 36 animals per analogue.