Between August 1989 and July 1990, California conducted a Mediterranean fruit fly eradication project in southern California which included repeated aerial applications of malathion bait to urban areas where approximately 1.6 million people resided. Concern about the safety of these applications prompted the California Department of Health Services to prepare a risk assessment. The current work presents the estimates of environmental levels of malathion and malaoxon, derived from mass deposition rates during application and monitoring of air. We estimated short and long-term malathion and malaoxon levels on outdoor surfaces, plants, and soils (0.1 and 1.0 cm mixing depth), using a simple first-order exponential decay model and literature half-life values ranging from three to nine days. Direct monitoring data were used to characterize short-term air levels. Average and upper-bound malathion levels immediately following an application were 0.091 microgram/m3 and 0.207 microgram/m3 in air; 22 and 52 mg/m2 on outdoor surfaces; 3.8 and 9.6 micrograms/g in plants; and 1.5 and 3.5 micrograms/g in soil (1 cm mixing depth). Malaoxon levels were 0.039 microgram/m3 and 0.110 microgram/m3 in air; 0.15 and 0.46 mg/m2, 0.03 and 0.09 microgram/g, and 0.01 and 0.03 microgram/g, respectively. Estimates of average and upper-bound levels over the duration of the eradication program were roughly one-third the immediate levels in all media. No field measurements were available for co-products other than malaoxon, for environmental persistence, or for concentrations on surfaces, soil, or plants after repeated applications. Because of limited monitoring data, and the likelihood that similar pest-eradication programs will be conducted, additional studies are warranted to more accurately characterize the environmental fate of malathion and human exposures after aerial application to residential communities.