In a large breeding colony of squirrel monkeys, a previous study demonstrated apparent universal infestation of adult animals with enteric trichomonads. The potential of these organisms to act as a source of experimental variability and the potential pathogenic effects of parasitism in this species stimulated this study of organism acquisition and treatment. Age of natural infestation with trichomonads was determined from results of microscopic examination and culture of fecal samples from infants of different ages. A majority of squirrel monkey infants showed first evidence of trichomoniasis at 2 to 4 weeks of age, with apparent 100% infestation by 8 weeks of age. Treatment of adult monkeys was investigated. In vitro techniques were utilized to determine sensitivity to metronidazole of a number of isolates. An effective regimen for treatment of adult monkeys was determined to be 25 mg/kg body weight of metronidazole given orally, twice daily for 5 days.