Mature pupae of Ceratitis capitata (Wiedemann) were irradiated with fast neutrons in a swimming-pool reactor, and flies which emerged 24-48 hours after irradiation were used in all experiments. The doses absorbed by the pupae were checked biologically with barley seeds and by the 32S (n, p) 32P reaction. Doses up to 1.9 krad did not adversely affect adult eclosion, and survival at 21 days of males and females was not significantly reduced by 1.9 and 1.1 krad, respectively. Irradiated females became infecund at a dose between 0.7 and 1.1 krad, and 98% sterility in males was achieved with 1.9 krad. Comparison with data for ⋎-radiation gives an RBE for the fast neutrons of 3-4 for female in fecundity and 4-5 for male sterility. Data from competitive mating experiments comparing irradiation with fast neutrons or ⋎-radiation suggested that fast neutrons affected male competitiveness less than did ⋎-radiation.