Prepupae of the Mediterranean fruit fly,Ceratitis capitata(Wiedemann), exhibited a significant mortality response under field conditions when exposed to concentrations of 5,000, 1,500,500, and 150 infective juveniles/em2of the insect parasitic nematode,Steinernema feltiaeFilipjev (Mexican strain). Additionally, concurrent field exposures of mature larvae (500 nematodes/em2) yielded significantly higher mortalities in the soil than in a vermiculite overlay. Fly mortality responses to nematodes produced in vivo or in vitro were not significantly different at the median exposure concentration. The estimated LCO50response of 38 nematodes/em2soil surface area indicates that these nematodes may offer a nontoxic alternative to soil treatments for Mediterranean fruit fly control programs.