Drosophila mercatorum is a sexual species that can reproduce parthenogenetically in the laboratory. A previous study showed that a natural population of D. mercatorum inhabiting the Kamuela garbage dump on the Island of Hawaii could produce both viable parthenogenetic adults and self-sustaining parthenogenetic lines. The present study deals with a second screen for parthenogenesis and an isozyme survey performed on natural populations of D. mercatorum and D. hydei caught in patches of Opuntia tuna about 10 kilometers from Kamuela. Both cactus-patch species produced viable parthenogenetic adults, but only D. mercatorum produced parthenogenetic females themselves capable of parthenogenesis. Moreover, D. mercatorum produced several "hot" lines characterized by high parthenogenetic rates, while all lines of D. hydei had a homogenous low rate. The parthenogenetic capacity of the cactus-patch D. mercatorum was lower than that of the garbage-dump D. mercatorum. Moreover, both the cactus-patch D. mercatorum and D. hydei had lower levels of polymorphism (26% and 22%, respectively) then the garbagedump D. mercatorum (44%), and both cactus-patch populations had heterozygote deficiencies with respect to Hardy-Weinberg equilibrium, unlike the garbage-dump population. Consequently, these data do not support the idea that decreased levels of heterozygosity in a sexual population increase the chance that sexual females will produce totally homozygous, parthenogenetic progeny.