Studies on the development of Ascaridia galli from X-irradiated embryonated eggs indicated that a complete absence of male worms is due to their elimination rather than to sex reversal. Elimination of males occurred no later than 10 days postinfection, the earliest when worms could be sexed by current methods. Results of hatching X-irradiated embryonated eggs in vitro indicated that under the conditions used potential male larvae were capable of freeing themselves from the eggshell. Absence of male worms after experimental infection of chickens with X-irradiated (5 to 80 kr) embryonated (infective) eggs has been reported (Varga, 1964; Ruff, Hansen, and Ostlind, 1965). Reports on other nematodes revealed a change in sex ratio but no complete elimination of males at low levels of radiation (5 to 20 kr). The purpose of this study was to determine if the absence of males was related to selective elimination of males or to sex reversal, and when the effect occurred. Sex reversal was hypothesized because physical factors such as temperature (Yoshikura, 1959) and chemical substances (hormones) (Hishida, 1965) have been shown to result in sex reversal in vertebrates. Radiation-induced sex reversal has been reported in a plant (Gomez-Campo and Casas-Builla, 1965). MATERIALS AND METHODS Eggs of Ascaridia galli were incubated (2 weeks) until they contained infective larvae. Some eggs were exposed to 10 kr of X-ray using a 150 kv Picker unit (tungsten target) while others served as nonirradiated controls. The techniques of measurement of dosage level and rate were those of Ostlind and Hansen (1966). Sixty-two, 7-day-old white rock chicks were each given per os 200 ? 20 irradiated or nonirradiated eggs according to experimental design. Intramuscular injections with 0.625 mg hydrocortisone acetate were given birds on the day of exposure to eggs and continued on days 3, 6, 9, 13, and 16 postexposure in order to increase worm burdens for better test comparisons. In two experiments birds were killed at various days postexposure (Fig. 1), and all lumen dwelling worms were recovered by flushing the intestines according to the method of Ackert and Nolf (1929). Received for publication 26 June 1967. * Contribution No. 401, Division of Biology, Kansas Agricultural Experiment Station. Supported by AEC Contract No. AT(11-1)-1209, publication COO-1209-5. Also, each intestine was slit and shaken vigorously in water to remove any larvae attached to the intestinal mucosa. All flushed and washed contents were combined and examined for worms using a dissecting microscope. The slit intestine was also examined to insure that all larval A. galli had been removed. Ruff et al. (1965) using the technique of Herlich (1956) showed by digesting the chicken intestine that the aforementioned methods were effective for recovering worms. Male worms were distingushed from females either by the presence of a preanal sucker (16to 65-day-old A. galli) or by the presence of a preanal swelling (10to 15-day-old worms) according to Ackert (1931). Worms younger than 10 days old showed no reliable character by which they could be sexed. Possibly, X-irradiation merely hindered hatching of potential male larvae. Studies on in vivo hatching of irradiated embryonated eggs were precluded because the small size of A. galli at 1 day postexposure prevents recovering adequate numbers and larvae cannot be sexed before the 10th day postexposure. This factor was investigated in vitro by comparing the percentages of embryonated (irradiated and nonirradiated) eggs hatching after 24 hr incubation at 40 C in an intestinal extract obtained from 6-week-old birds. The extract was prepared from a portion of the intestine beginning at the gizzard and extending approximately 5 inches posterior to the bile duct. The contents were pressed out, centrifuged at 1,000 RPM for 5 min and the supernatant filtered twice using two thicknesses of No. 2 Whatman filter