The relative efficacies of pyrantel tartrate and of pyrantel citrate against Oesophagostomum sp in swine were evaluated in a controlled-critical study and the efficacy of pyrantel citrate in a field trial. In the controlled-critical study, pigs naturally infected with Oesophagostomum dentatum were either not treated or were treated with pyrantel citrate or pyrantel tartrate at a dosage of 510 mg of free pyrantel base/kg of feed. Six days later, the pigs were necropsied, adult O dentatum was recovered and counted, and fecal samples were examined for helminth eggs. The efficacies of pyrantel citrate and pyrantel tartrate were each 100% based on fecal egg counts and numbers of adults at necropsy. The field trial was conducted in a similar manner except that pyrantel citrate only was tested against a control group on the basis of fecal egg counts made both at the beginning and at the termination of the trial. In this study pyrantel citrate was found to reduce Oesophagostomum sp egg counts by 89.4%.