Hydroprene [ethyl (E,E)-3,7,11-trimethyl-2,4- dodecadienoate], an insect growth regulator (IGR) "or juvenile hormone analog, was applied to substrata where oriental cockroach nymphs, Blatta orientalis, were reared. A detailed study of the morphology of B. orientalis external genitalia indicated that hydroprene affected the development of these structures. Both the male and female genitalia were malformed after exposure to hydroprene. The epithelial cells of the male genitalia, particularly of the right and left phallomeres, were more sensitive to the IGR than that of the female. A study on the configurational fitness of genitalia, or coupled genital structures, at copulation revealed that adults with malformed genitalia were unable to initiate mating and to copulate successfully. Wings of the male were ranked into four degrees of malformation, with o being normal and 3 being the most malformed. There was a 95% correlation between wrinkled wings (ranked 1 to 3) and male genital malformation. Wings rated 2 and 3 were 100% correlated with genital malformation. Approximately 93% of hydroprene-exposed nymphs were sterilized when they molted to the adult stage. This was highly significant in comparison to the normal (P<0.001). Late instar nymphs were more sensitive to hydroprene than early instar nymphs. The development of ovaries, testes and accessory glands was morphologically inhibited in the adultoids or sterile individuals. The majority (99%) of the !GR-treated nymphs underwent molts and metamorphosis, which did not differ significantly from the controls (P>0.25). An unusual post-adultoid molting occurred in approximately 39% of the adultoids, which resulted in significantly higher mortalities than the normal (P<0.001). Mean wet body weights of adultoids were significantly heavier than that of the normals (P<0.05). / M.S.