Parasitological analysis was carried out during 1982 and 1983 in 3,076 children aged under 7 years, who were enrolled in 52 day nurseries supported by Municipal Prefecture of São Paulo and 50 food handlers (cooking employees) working at 18 day nurseries. The prevalence of Giardia lamblia infection was studied according to sex, age group and seasonal variations, and attained 32.80% among children and 4.0% among adult population. The occurrence of giardiasis was most frequent in age group corresponding to 1-4 years old. The study of seasonal variations showed that, in 1982, the prevalence of G. lamblia was high in the autumn and in the spring and, in 1983, giardiasis was more prevalent in the autumn and in the summer. Statistical analysis indicated non association between sex and prevalence for Giardia infection, and there was also no significative difference on the distribution of G. lamblia prevalence in the four regions analysed in the present study (p > 0.05). The most probable mode of acquisition of Giardia infection, in the nurseries analysed in the present study was person-to-person, as factors, like environmental contamination and others, associated with the spreading of giardiasis through water, reported in other countries, are unlike from those verified in our Municipality. On the other hand, there is evidence of no significant role of food handlers in Giardia transmission, because the rate of infection in this group was very low.