A strain (BKPM) of Aedes aegypti from Bangkok, after some mass selection with permethrin, showed an initial resistance level to permethrin of about 20‐fold compared with a standard susceptible strain. The strain was subjected to a programme of single‐family sib selection, concurrently with further mass selection. The F1 and F2 results of single‐family sib selection established that BKPM was heterogeneous for resistance to permethrin. The most resistant family in the F1 and F2 was inbred to an F3. The results indicated that this family was homogeneous. A strain BKPM3 was established from this family and found to have a resistance level of about 30‐fold. Despite the heterogeneity of BKPM, five generations of mass selection failed to raise the resistance level to permethrin. The selection procedure was then changed, in as much that the insects were sorted at the end of the 60‐min exposure instead of after the 24‐h holding period, and concentrations were chosen to give a higher selective effect. This resulted in an almost immediate rise in resistance to a level similar to that of BKPM3, and subsequent mass selection failed to raise this further. The possible effects of the changes in selection procedure are discussed.