Diphyllobothrium pacificum, a large tapeworm of seals, is often found as an accidental parasite of man in Peru. The main symptoms are abdominal pain, meteorismus and flatulence, and diarrhea. We report the results of 32 single-dose (10 mg/kg weight) treatments with Praziquantel (Cesol R, EmBay 8440) in 31 patients. One of these patients was shown to be cured but presented a reinfection 270 days later, and required retreatment. Praziquantel was effective in all cases. All patients were clinically cured. No ova were found in stool examinations at 30, 60, and 90 days after treatment, while pretreatment stools were positive for ova with all the techniques used. The antiparasitic effect was evident in 11 cases in which segments of the tapeworm were eliminated within the first 3 days of treatment: the fragments were all partially digested or autolysed. A treatment scheme incorporating a purge may be best, to recover the parasite in its entirety and thus ascertain cure with no repeated controls.