The toxicity of fipronil and dieldrin was determined in one susceptible laboratory strain and seven insecticide-resistant field-collected strains of Blattella germanica (L). The Zo960302 and Ga021001 strains were 1,270- and 2,030-fold resistant to dieldrin and 15- and 14-fold resistant to fipronil. The Su960304 and Od010803 strains were 15- and 13-fold resistant to dieldrin and two- and four-fold resistant to fipronil. Three strains showed no or a low level of resistance to dieldrin and fipronil. Crosses were performed between the susceptible strain Danish Pest Infestation Laboratory (DPIL)-SUS and the resistant strains Zo960302 and Su960304 and resistance to dieldrin and fipronil were intermediate compared with the susceptible and the resistant strains. Backcrosses to both of the parental strains showed cosegregation of dieldrin and fipronil resistance. The toxicity of dieldrin and fipronil was correlated when compared at LD₅₀, and 93% of the observed variation in LD₅₀ of fipronil can be ascribed to variation among predictions based on the value of LD₅₀ of dieldrin. The frequency of the A302S substitution in the resistance to dieldrin (Rdl) gene in the highly dieldrin- and fipronil-resistant strains Zo960302 and Ga021001 and the moderately resistant Su960304 was 0.97, 1.0, and 0.38, respectively. We consider the connection between the frequency of the Rdl mutation and dieldrin and fipronil resistance a causal connection and not merely a coincidence.