Occupational exposure to organophosphate pesticides and its effects on the concentration of erythrocyte cholinesterase in the rural population of Chiapas, Mexico, are described. The authors surveyed agricultural production and pesticide use was surveyed among 199 campesinos (peasants) in three communities that used various agricultural production systems. The authors measured the concentration of the cholinesterase enzyme in blood samples obtained from 65 campesinos before and after exposure to the insecticide. The authors established a comparison value for the population that was not exposed occupationally. The exposure values of the enzyme concentration were significantly lower than preexposure values (p = .00001) and reference group values (p = .0008). Individuals in the community characterized by subsistence production had significantly lower levels of the enzyme than individuals in the other two communities (p = .01). This result suggested that a greater risk of adverse health effects existed among the poorest communities.