Stool samples from a population-based cohort of mothers and children living in Leogane, Haiti were tested for Cyclospora cayetanensis from January 1997 through January 1998. Data on gastrointestinal symptoms were also collected. During the winter months of January to March, the infection was detected in 15-20% of the persons sampled. Most infections did not appear to be causing diarrhea and most infected persons had few oocysts detectable in concentrates of stool. The infection appears to have marked seasonality, with highest rates during the driest and coolest time of the year. It may be that in this tropical setting, high summer temperature is the critical environmental factor that influences the seasonality of infection. This study demonstrates that Cyclospora infections in Haiti are common in the general population.