Populations of the green peach aphid,Myzus persicae(Sulzer), and associated weed hosts were monitored continuously from April 1978 to March 1979 on the floor of three peach orchards and the bank of a drainage ditch. Aphid populations at all sites peaked in spring and declined in summer. Although natural enemies were most active during the period of peak aphid abundance, they were not numerous enough to account for population declines at all locations. In 2 orchards, significant positive correlations were found between green peach aphid populations and the abundance of preferred host plants. No such correlation was found in the third orchard, where aphids were less abundant and patchily distributed, or in the ditch bank, where pathogens and climatic factors may have limited the aphid population.