Abstract: The duration of fipronil topical application toxicity of twelve strains of German cockroaches, Blattella germanica (L.), was determined. The Saman strain was collected in 2002, from infested Habitable Convened and the Bustan-7, Bustan-8, Bustan-10, Kouye-Pezeshky, Shahmorady, Zanjan and Kouye-Tehran strains were collected in 2001, from different infested student dormitories and the Fayyazbaghesh, Mogtameh and Kouye-Dokhteran strains were collected in 1997 from two infested hospitals and one student dormitory, respectively in Tehran, Iran. In the topical application bioassay, the average LD50 of susceptible strain was 1.8, 1.3 and 0.96 ng per insect after 24, 48 and 72 h, respectively and the average LD95 was 3.4, 2.6 and 2.2 ng per insect after 24, 48 and 72 h, respectively. As with the susceptible reference strain, where LD50 was halved from 24 to 72 h, the LD50 of fipronil decreased with time in the field populations. LD50 varied > 3-fold from 1.2 to 3.6 ng per insect at 24 h and almost 2.5-fold from 1.2 to 3.0 ng per insect at 48 h. At the end of the bioassay at 72 h, LD50 varied from 0.94 to 2.5 ng per insect, which is 1.6- and 2.6-fold higher than the standard susceptible value of fipronil and the steep slopes of dose-response curves indicated that the field population of these German cockroach strains was homogenous in response to fipronil. All German cockroach strains showed a similar susceptibility or lower tolerance (1.5 to 2.6-folds) for fipronil compared with the susceptible laboratory strain and the steep slopes of dose-response curves indicated that the field population of these German cockroach strains was homogenous in response to fipronil. These results indicate that the fipronil was relatively slow-acting in topical application bioassay, with LD50 values decreasing until 72 h and becoming stable thereafter.