Effects of atmospheric ammonia at 69.4 mg/m3 (100 ppm) on productive performance and respiratory tract health of young pigs (starting body weight averaged 7.5 kg) experimentally infected with Ascaris suum (50,000 embryonated ova administered by gavage when pigs were 5 weeks of age) were studied in 5 trials of 4 weeks each (when pigs were 5 to 9 weeks of age). Effects of atmospheric-ammonia exposure and ascarid infection on growth were additive. Compared with controls, percentage reductions in average daily gain were 32%, 28%, and 61% for ammonia-exposed, ascarid-infected, and combined ammonia plus ascarid groups, respectively. Ammonia exposure or ascarid infection alone depressed feed disappearance by 18%. Effects of the 2 factors were additive, resulting in a 35% reduction in feed disappearance. Pigs exposed to the combined factors had an average gain/feed ratio of 0.518, which was less than that of control pigs (0.546), but was greater than that of pigs exposed to atmospheric ammonia (0.489) or pigs infected with ascarids (0.501) alone. Liver scarring, due to larval migration, was not affected by ammonia exposure. Larval migration through the respiratory tract was not confirmed histopathologically in pigs killed 4 weeks after inoculation. A supplementary experiment was conducted which demonstrated that residual evidence of previous pulmonary larval migration was present 2 weeks after inoculation.