Five studies were carried out in children younger than 4 years old attending regularly day care centres in order to determine the intestinal parasitization by Cryptosporidium and Giardia. The percent of children parasitized by Cryptosporidium was 10% (17 children out of 170 studied). Cryptosporidiosis was more frequent in winter than in other seasons (difference statistically significant). Giardia intestinalis, which was the most frequent parasite, was identified in 25.3% (43 children out of 170 studied). Giardiasis was more frequent in autumn and in the low SEL (difference statistically significant in both cases). The frequency of giardiasis increased from 8% in children studied during 1-2 seasons, to 34% in children studied for 3-5 seasons (difference statistically significant), whereas the frequency of cryptosporidiosis did not show variation with increased number of specimens studied per child. The 35% of children parasitized by Cryptosporidium sp. showed gastrointestinal symptoms but only 14% of children with Giardia intestinalis.