Eight experiments were conducted, involving 179 pigs, and dealing with the efficacy of thiabendazole in the prophylaxis and therapeusis of Trichinella spiralis infections in swine. A single subcutaneous dose of thiabendazole at 150 or 200 mg/kg effectively suppressed the enteral phase of trichinosis, when given 48 hr before inoculation. Smaller dosages gave variable protection, and in two instances allowed the development of an infertile enteral population. Immature trichinella were more sensitive to thiabendazole therapy than were adult worms. Immature worms were eliminated from the gut by a single dose of 150 mg/kg; lower dosages were not tested. Adult worms were usually eliminated by doses of 25 mg/kg given twice daily (for 2 to 3 weeks), beginning 1 or 2 weeks after inoculation. During the invasive phase of trichinosis, beginning 1, 2, or 3 weeks after inoculation, the number of trichinella larvae developing in the musculature was greatly reduced by twice-daily doses of 25 or 75 mg/kg. Under these conditions, the effect of the drug on the parenteral phase may have been enhanced by its effect on the enteral phase. When treatment was begun at 5 to 8 weeks after inoculation, doses of 25 mg/kg, twice daily, had no detectable effect on larvae in the muscle of pigs, but twice-daily doses of 75 or 150 mg/kg reduced the number and viability of the larvae. The last dosage, amounting to 300 mg/kg/day, was toxic to the pigs. In histopathological sections some of the larvae appeared degenerate and presumably had been killed by treatment. Some of the larvae that survived treatment of the host were apparently incapable of withstanding peptic digestion; other larvae withstood digestion but were noninfective to mice. Whether such larvae are in fact moribund in the host muscle is not known. Data obtained from untreated control pigs indicated that a gross expulsion of enteral trichinella occurred 3 to 5 weeks after inoculation. The expulsion was accompanied by a reversal in the sex ratio of the worms, so that the small residuum of worms consisted mainly of males. Pigs exposed to 10,000 larvae per kilogram of body weight developed no fever and only moderate eosinophilia. Preliminary studies have demonstrated the prophylactic and therapeutic efficacy of thiabendazole against Trichinella spiralis in domestic swine, Sus scrofa (see Campbell and Cuckler, 1962a, 1962b; Zimmermann, 1964). The present paper presents: (1) further prophylactic studies; (2) further therapeutic studies; and (3) incidental observations on the biology of trichinella in swine. MATERIALS AND METHODS In these studies 179 pigs were used. The pigs were crossbred (Hampshire-Yorkshire) and were farrowed in concrete pens on a research farm; they were not more than 8 weeks old, and, apart from occasional light ascaris infections, they were essentially free of parasites. Throughout each experiment they were housed in concrete pens, usually with a raised slat floor. They had unrestricted access to water and commercial hog feed. Except in Experiment 2 the pigs of each experimental group were housed together. The experimental groups of pigs were balanced with respect to sex, weight, and litter of origin. Received for publication 17 May 1965. Pigs were exposed to infection by inoculation of larvae harvested from infected mice. The larvae were suspended in 5% gelatin and introduced deeply into the esophagus by means of a syringe and blunt rubber catheter. Throughout the investigation, the inoculum was kept constant at approximately 10,000 (? 10%) larvae per kilogram of body weight. Since the number of larvae inoculated bore a fixed ratio to host body weight, the initial weight range of the pigs in many experimental groups can be calculated from the data in the tables. Where the recovery of worms was less than 1% no such calculation can be made, but the weight of any one group of pigs may be taken as typical of the other groups in the same experiment. Parenterally administered thiabendazole was injected deeply, by means of an 18-gauge hypodermic needle, i to the connective tissue immediately posterior to the parotid glands. Thiabendazole was given orally by means of a blunt rubber catheter inserted into the esophagus. Where thiabendazole was given as the hydrochloride salt, the dosage is expressed in terms of the base. Tissues collected for histological examination were fixed in formalin and stained with hematoxylin and eosin. Rectal temperature was measured either by mercury thermometer or by thermistor. Relative eosinophilia was estimated by differential count of white blood cells, using standard