Cryptosporidium parvum is an endemic, zoonotic coccidian parasitosis that is highly prevalent in third-world countries where waterborne fecal contamination of food and drink or person-to-person contact with oocysts are the most common methods of transmission of the enteric protozoan. This type of transmission of the parasite made the villages around Lake Atitlan, Guatemala a unique site to compare environmental risk factors with the level of Cryptosporidium infections in the local residents. The study was carried out in two villages, San Antonio Palopo and Santa Catarina Palopo, located in the highlands above the shores of the lake. Smears from stool specimens of patients with gastroenteritis were processed using Kinyoun's modified acid-fast stain and observed with light microscopy. Of the 100 residents examined from the two villages, 32% had Cryptosporidium infections. Female children had the highest prevalence of infection (44% in San Antonio Palopo and 46% in Santa Catarina Palopo, p<0.05), and they also had significantly higher infection rates than males, 50% vs. 17%, respectively. The prevalence rate was not influenced by the season of the year or by the location of the residents. We found differences in prevalence rates due to age and gender, and we suggest that the high infection rates of specific groups are associated with their exposure to the contaminated water supply from Lake Atitlan.