Abstract: The duration of fipronil WHO glass jar method toxicity against twelve strains of feral German cockroaches, Blattella germanica (L.), was determined. In the WHO glass jar bioassay, the average LT50 of susceptible strain was 16.4,14.3,12.4 and 11.3 exposure minutes after 24,48,72 and 96 h, respectively and the average LT95 was 20.3,19.9,19.5 and 19.1 exposure minutes after 24,48,72 and 96 h, respectively. As with the susceptible reference strain, where LT50 was halved from 24 to 96 h, the LT50 of fipronil decreased with time in the feral German cockroach strains. LT50 varied > 8-folds from 16.2 to 24.7 exposure min at 24 h, 8.4 folds from 14.4 to 22.8 min at 48 h and almost 8.8 folds from 12.5 to 21.3 exposure minutes at 72 h. At the end of the bioassay at 96 h, LT50 varied from 11.6 to 19.7 exposure minutes, which is 1.0 and 1.7 folds exposure min higher than the standard susceptible value of fipronil. All German cockroach strains showed a similar susceptibility or very low tolerance (1.5 to 1.7 folds) to fipronil compared with the susceptible laboratory strain and the steep slopes of time exposure-mortality curves indicated that the feral German cockroach strains was homogenous in time exposures to fipronil. These results indicate that the fipronil was relatively slow-acting in WHO glass jar method bioassay, with LT50 values decreasing until 96 h and becoming stable thereafter.