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Abstract Forty-eight crossbred barrow pigs naturally exposed to Strongyloides ransomi and Ascaris suum were used in a 23 factorial experiment to study; (1) the effects of pyrantel HCl (0 and 0.0715 g/kg of feed fed continuously for 42 days); (2) levels of dietary crude protein (higher, 18-16-14%; and lower, 15-13-11%); and (3) sequence of experimental exposure to 3 parasite genera (S. ransomi, A. suum, Oesophagostomum spp.; and A. suum, S. ransomi, Oesophagostomum spp.) on performance of pigs weaned and placed on experiment at 8.9 kg average weight. After 42 days, rates of gain of pigs on the lower protein diet with and without pyrantel HCl were not different; however, pigs fed pyrantel HCl in the higher protein diet gained 20% faster (P<.10) than those not fed pyrantel HCl. Atfer 77 days of feeding, this interaction had disappeared; gains of pigs on both protein levels were similar (P>.10), but pigs which had received pyrantel HCl had gained 8% faster (P<.05) than those without pyrantel HCl. Pigs fed the higher protein diet were more efficient in conversion of feed into weight gain the first 42 days (P<.01) but, by the time the pigs reached market weight, an interaction of pyrantel HCl and dietary protein level masked any primary effects. At 42 days the average S. ransomi egg per gram of feces (EPG) count from pigs not fed pyrantel HCl was 2.9 times that of pigs fed pyrantel HCl. At the same time, the average S. ransomi EPG count for pigs exposed first to S. ransomi was 7.8 times greater than that for pigs exposed first to A. suum . Pyrantel HCl was associated with an 85% reduction (P<.01) of A. suum EPG counts at 77 days, however, the 44% reduction at market weight lacked significance (P>.10). The S. ransomi EPG counts had no bearing on feed efficiency at 42 or 77 days, but Ascaris and Oesophagostomum EPG counts had a negative correlation (P<.05) with efficiency at 77 days. The EPG counts at market weight for S. ransomi and Oesophagostomum spp. were positively correlated with the respective worms (P<.01); however, such was not the case with A. suum since less than 10% of the variation in egg production could be associated with the number of worms present (P>.10). Pyrantel HCl effectively reduced the average numbers of S. ransomi and A. suum up through 77 days; however, such apparent differences did not exist at market weight. Exposure to S. ransomi first was associated with greater daily feed intake (P<.10); a decreased rate of efficiency (P<.05); and a greater number of S. ransomi and Oesophagostomum spp. worms (P<.10) at market weight. Copyright © 1972. American Society of Animal Science . Copyright 1972 by American Society of Animal Science.