Abstract.Permethrin selection of DDT resistantAnopheles stephensiListon mosquito larvae produced a reduction in susceptibility to knockdown (2 h exposure) of 17‐fold, but only 1.6‐fold to kill (24 h exposure). Genetic analysis, incorporating visible mutant markers, was interpreted as indicating that, through multigenic inheritance, several interacting genetic factors were collectively responsible for reduced larval susceptibility to knockdown. These were maintained together only under selection pressure, as the effect was lost quickly in the absence of selection or with outcrossing. The 30–40‐fold DDT‐resistance found in the parental strain was barely altered by permethrin selection, suggesting no relationship with the major source of DDT resistance. This was confirmed in single family studies. Some evidence for an additional tolerance to DDT was found to be associated with reduced larval susceptibility to p