The mosquito adulticide, malathion [O, O-dimethyl S-(1,2-dicarbethoxyethyl)phosphorodithioate] was studied in terms of deposition as a function of distance from the point of application in an open area. The parameters, mass deposited, droplet abundance, and droplet size (mass median diameter), were determined at distances 5, 25, 100, and 500 m from the spray head. Five separate tests yielded decreasing mass of malathion deposited with distance: i.e., 33.36, 16.75, 15.72, and 2.10 ng/cm2 were found at distances 5, 25, 100, and 500 m, respectively. Malathion mass deposited differed significantly between the 500 m site and the three closer sites (df=3; F-value=3.42; P<0.05). Although droplet abundance decreased with distance from the spray head, no significant differences were detected between distances. Significant differences in droplet size were found between the 500 m site and the remaining three sites using orthogonal contrast analysis (df=1; F-value=9.5; P=0.009). The validity of hypothetical estimates (assuming 100% deposition and homogeneous distribution within the target zone) was evaluated by comparison to field results. Compared to an expected field concentration of 577 ng/cm2, empirical data yielded a maximum of 33.4 ng/cm2 or 5.8% of the expected mass 5 m from the spray head.