A longitudinal study of Ascaris lumbricoides reinfection over a 9-10 month period was carried out in a wastewater irrigated fanning region in Central Mexico (September 1989 - May 1991). The study assessed the relationship between different levels of exposure and its components, in the context of wastewater irrigation, and the prevalence and intensity of Ascaris infection. Three farming populations were studied: (1) those who irrigated with untreated or raw wastewater; (2) those who irrigated with wastewater that had sedimented during passage through a storage reservoir (a form of partial treatment); and (3) those who lived in a rain-fed area. Individuals' exposure to Ascaris eggs was estimated by in-depth interviews (characterising the frequency and type of contact with wastewater). Wastewater contact through agricultural activities was also estimated by structured observation. Predisposition to Ascaris infection was largely a consequence of behavioural and environmental factors that caused wastewater contact. Overdispersion of Ascaris intensity in the study population was not age- or gender-dependent. Over the 12-month monitoring period, untreated wastewater had a mean concentration of 96 Ascaris egg/litre and sedimented wastewater a concentration of <1 Ascaris egg/litre. Contact with untreated wastewater during various activities was associated with differing degrees of excess risk of Ascaris infection in the respective groups: -crop irrigation: 3-fold risk among children -chilli production: 5-fold risk among men and higher intensity infections -tending livestock: 4-fold risk among women -consumption of crops irrigated with wastewater: 2-fold risk in men and children and higher intensity infections in children -sweeping the yard: 5-fold risk in women Contact with sedimented wastewater during play was associated with more than a two-fold risk in Ascaris infection among children, and during maize production, with higher intensity infections among men. The nematode egg guideline of ~l egg/litre is adequate to protect the health of farmers using wastewater in agriculture, but is not sufficient to protect children. Any future modifications of the guideline must consider this.