Variation in tolerance to ammonium chloride among four strains of Aedes aegypti was investigated. Significant heterogeneity between strains was found and it was possible to more than double the tolerance of one strain by three generations of selection. A reduction in pupal size was observed in the survivors of exposure to ammonium chloride, the effect being more marked in the more susceptible strains. The evidence for a genetic component of tolerance is discussed in relation to possible adaptation to sewage-polluted waters.