Technical Abstract: In recent years, the pest status of Liriomyza trifolii and L. huidobrensis has changed in california, as well as other areas of the world. In california, L. huidobrensis has become the predominant Liriomyza species in valleys along the central coast, while L. trifohi remains the predominant species in southern California. To investigate possible reasons for species displacements, differences in host plant use and reproductive success of intraspecific populations were examined for Liriomyza trifolii and L. huidobrensis from both central and southern California. The southern L. trifolii fed, oviposited and reproduced successfully on all five hosts tested, but the central population fed significantly less on all hosts and was restricted to reproducing only on pepper. With the exception of pepper, southern L. trifolii had significantly greater larval survival on all hosts. In contrast, the central L. huidobrensis population had greater reproductive success than the southern population of that species on all hosts plants tested. However, pepper was not a suitable host for either L. huidobrensis population. Both species showed positive assortative mating, with homotypic mating occurring more frequently than heterotypic mating; however, the difference between L. trifolii populations was much more pronounced than between L. huidobrensis populations. These data indicate that central and southern California populations of each species are at least distinct biotypes, and their interactions could be a mechanism for incipient speciation.