This study assessed the risk factors for Giardia intestinalis infection in an agricultural population in Mexico. Exposure groups included 2,257 individuals from households exposed to untreated wastewater, 2,147 from a group using the effluent from a series of reservoirs, and 2,344 from rain-fed agricultural villages. Stool samples were collected from 6,748 individuals. Wastewater samples were tested for fecal coliforms/100 ml and Giardia sp. cysts/L. Untreated wastewater samples contained 10(8) fecal coliforms/100 ml and up to 300 Giardia sp. cysts/L. Hydraulic retention (3-7 months) in the reservoirs, however, provided an improved effluent quality (10(1)-10(4) fecal coloforms/100 ml and < or = 5 Giardia sp. cysts/L). Children 1-14 years of age had the highest prevalence of infection (20%). Data showed marginal associations between storing drinking water in unprotected containers and lack of facilities for feces disposal and the risk of infection (odds ratios [ORs] = 1.76 and 1.19, 95% confidence intervals [CIs] = 0.95-3.23, and 0.97-1.45, respectively). Individuals purchasing vegetables at the city market had higher rates of infection than those buying at the village shop (OR = 2.49, 95% CI = 1.00-6.17). No excess risk was found in individuals exposed to untreated wastewater compared with controls (OR = 1.07, 95% CI = 0.84-1.36); the group using reservoir water was not different from the controls (OR = 1.22, 95% CI = 0.94-1.58). No risk from agricultural activities was detected (OR = 0.83). This pattern of infection may be addressed by primary health care and wastewater treatment.