Population ecology ofAphis neriiB.d.F.on oleander (Nerium oleanderL.) in northern California was assessed. A native generalist,Lysiphlebus testaceipesCresson, was the only primary parasite reared fromA. nerii, and in certain cases, it was able to control the host population. However, parasitization byL. testaceipeswas an inverse-density-dependent factor and thus not a significant mortality factor at high aphid population densities. The latter populations were apparently regulated through intraspecific mechanisms (e.g., competition). Outbreaks ofA. neriioccurred only at urban sites where plants were regularly pruned and watered. These cultural practices resulted in an increased proportion of new, actively growing terminals (preferred byA. nerii) which led to increased density of aphids feeding on such terminals. Relevance of these studies to insect ecology in urban environments is discussed.