Aedes sierrensis (Ludlow), the western tree-hole mosquito, is the first species of the pulchritarsis-varipalpus group of the subgenus Ochlerotatus which has been shown to have the masculine genotype modified by thermal pressure applied to developing larvae. A rearing temperature of 30.0 °C. during instars 1-3 and the first half of 4, and 31.0° during the last half of instar 4 will modify the degree of masculinity of the palpi, antennae, oral stylets and testes. Modified males are impotent because the genital section fails to rotate to the copulatory position.