The γ-amino butyric acid (GABA)-gated chloride ion channel in the insect central nervous system is the target of cyclodiene and phenylpyrazole insecticides. Resistance to dieldrin has been reported in several insect species and was associated with a point mutation (Ala²⁸⁵ to Ser substitution) in the M2 transmembrane domain of the GABA-gated chloride ion channel (the resistant to dieldrin [Rdl] gene). A partial Rdl gene sequence was reported previously in specimens of the cat flea, Ctenocephalides felis (Bouché). Because the presence of the Rdl gene mutation coincided with a reduction in susceptibility to fipronil in some insect species, it has been inferred that a similar association may exist in cat fleas. The Rdl gene sequence was evaluated in 20–50 fleas each from six cat flea strains shown previously to be fully susceptible to fipronil. Total DNA or RNA from fleas was extracted using a commercial kit, and the sequence encompassing the single nucleotide polymorphism (SNP) position Rdl was amplified by polymerase chain reaction (PCR) or reverse transcription-PCR. Amplification products were sequenced on both strands. All tested strains were homozygous for the mutant allele (T nucleotide at SNP position); amino acid sequencing demonstrated the Ala²⁸⁵ to Ser substitution. The results of this study indicated that the Rdl gene mutation was uniformly present as homozygous alleles in strains of fleas that have been shown to be fully susceptible to topically applied fipronil and that the efficacy of fipronil against cat fleas was not impacted by the Rdl gene mutation.