The esterase B gene is amplified 32-fold in an organophosphorus insecticide resistant strain, Shinjuku, of Culex pipiens molestus. The amplified gene was revealed to cluster in single extended chromosomal region (ECR) of chromosome 2 by fluorescence in situ hybridization on salivary gland chromosomes. The ECR appeared to be homogeneously non-staining with DAPI, orcein or Giemsa stains. In the F1 heterozygotes between Shinjuku and an insecticide susceptible strain, chromosome 2 showed an apparent asynapsis in the ECR derived from Shinjuku, but otherwise paired well. The results suggest that the process of DNA amplification does not involve large chromosomal rearrangements. Restriction map of the co-amplified sequence with esterase B gene in Shinjuku strain is unique compared to those reported in C. pipiens complex, supporting an independent origin of the DNA amplification.