Abstract.Mixtures of chromosomal forms A, B, C and D in natural populations ofAnopheles dirusPeyton&Harrisonsensu latoin Thailand show significant positive values of Wright's fixation index for six enzyme‐electromorph loci. The mean value ofFISover all loci was found to be +0.28 (SD 0.02), with a range of +0.57 (Odh) to +0.10 (Idh‐2). Partitioning electromorph data for the chromosomal forms reduces the meanFISto 0.03 (SD 0.01), which suggests that positive assortative mating is a characteristic of each form. This supports the hypothesis that the chromosomal/electrophoretic forms A, B, C and D represent four distinct biological species within theAn.diruscomplex.An example is given of the use of enzyme electromorphs as a means of vector identification during a malaria entomological field study involving a mixture ofAn.dirusspecies A and D. Electromorph identifications of 323 sp. A and 161 sp. D were more than 98% correct when cross‐referenced to specific DNA p