The sensitive developmental period of last-instar German cockroaches, Blattella germanica (L.), to topical applications (10 micrograms/microliters) of fenoxycarb and hydroprene was determined. Developmental duration of last instars treated with fenoxycarb on days 1, 3, or 6 was 9-11 d longer than the development of nymphs treated with hydroprene or acetone. In addition, fenoxycarb caused mortality in 59% of the nymphs treated on day 6. A high level of wing twisting and sterility was observed among female and male cockroaches treated with fenoxycarb or hydroprene on days 1, 3, or 6 of the last stadium. Nymphal production by adults treated with juvenoids on day 9 of the last stadium (3-4 d before eclosion) was not affected. Our findings show that the sensitive developmental period for juvenoid-induced effects in last-instar B. germanica is limited to the first half of the stadium.