The present clinical trial was conducted to determine the efficacy and tolerance of oxantel(CP-14, 445), one of pyrimidine derivative, in the treatment of Trichocephalus trichiurus infection. One hundred eighty two cases who harboured T. trichiurus were treated with oxantel in a single dose of 10 to 25 mg/kg of body weight. For the evaluation, the negative conversion rate and egg reduction rate were calculated before and 22 days after the treatment. The results obtained were as follows: The prevalence rate of T. trichiurus infection in present study was 64.2% by examining 1,536 fecal specimens collected. The overall negative conversion rate was 66.5%, while the highest rate(93.3%) was obtained in the group treated with 20 mg/kg of oxantel (Group III). The lowest rate(56.6%) was observed in Group I (10 mg/kg). The overall egg reduction rate on present study was 90.3%. The highest rate, 100% of egg reduction was also obtained in Group III (20 mg/kg). From the observation of 1,244 expelled worms, it was known that 45.0% of collected worms were expelled on the first of post-treatment day, and male and female worms were about the same in number(1.03 : 1). Average worm burden per capita was 11.1 in this observation. The medication was readily accepted and tolerated by all the treated cases. On the hemograms, urinalysis and liver function tests, no significant differences were noted before and after the drug administration. Side reactions were so mild and transient that none required specific treatment.