Significantly fewer adult worms were recovered on days 7, 9, 11 and 13 postinfection (PI) from the small intestine of CD-1 Swiss white mice infected with 250 Trichinella pseudospiralis than mice infected with 250 Trichinella spiralis. However, the distribution of adult worms between the anterior and posterior halves of the small intestine was similar in both species on all days during infection. Adult T. pseudospiralis deposited significantly fewer migratory larvae in vitro on day 7 PI than did adult T. spiralis. Significantly fewer larvae were isolated from the muscles of mice infected with T. pseudospiralis than from mice infected with T. spiralis. Although enteritis was detectable earlier in mice infected with T. pseudospiralis, mice infected with T. spiralis had significantly greater levels of enteritis after day 7 PI than those infected with T. pseudospiralis. Injection of the host with cortisone had no effect on the rate of in vitro larviposition on day 7 PI by adult T. spiralis, but such treatment significantly increased the rate of in vitro larviposition by adult T. pseudospiralis. Injection of mice with cortisone altered the distribution of adult worms of both species in the host small intestine on days 11 and 13 PI. As the age of the host increased, the numbers of adult T. spiralis and T. pseudospiralis recovered were unaffected, but the distribution and fecundity of adult worms of both species were altered. The numbers of adult T. pseudospiralis and T. spiralis were unaffected by the sex of the host. After day 9 PI fewer adult T. pseudospiralis and T. spiralis were located in the anterior half of the intestines of female than of male mice. The fecundity of adult worms of both species was reduced in female mice. Adult T. pseudospiralis incubated in vitro deposited greater numbers of migratory larvae when incubated at 42 C than they did when incubated at temperatures above and below 42 C.