The Amejeiras strain of Blatella germanica (Dictyoptera: Blattellidae) kept in lab conditions since 1994 and susceptible to arpocarb, malathion and cipermethrin insecticides, was studied. This strain was treated with 4 diflubenzuron (Dimilin OMS 1804) concentrations supplied by feeding nymphs at their first stage of development, with the purpose of determining their effect on the survival, phenotype and period of nynmphal development of this species. The dosage-mortality line of regression was determined and the values of CE50 = 0.042% and CE95 = 0.357% were obtained. Sublethal effects were observed in the nymphs treated with the 4 tested concentrations whereas the morphogenetic effects in adult insects only occurred when they were exposed to higher concentrations (0.25% and 0.4%). The nymph development stage showed a highly significant increase when the diflubenzuron concentration was raised. (F = 11.64; p < 0.001).