An evaluation is made of the utility for the diagnosis of giardiasis of an enzyme immunoassay (EIA) that detects the GSA65 specific antigen in feces, as compared with the formalin-ether sedimentation test. Within the context of a longitudinal study on intestinal parasitization, 147 fecal samples from children attending 7 day care centres in the city of Salamanca were studied. The feces were examined a few hours after collection by the sedimentation technique and conserved in 10% formol. They were later studied by the EIA method. G. intestinalis was detected by microscopy and/or EIA in 26 of the 147 fecal samples (17.7%). Both assays were positive in 22 of the 26 cases detected (84.6%) and the remaining 4 cases were positive only by EIA (4/26; 15.4%). The EIA assay, using visual reading and very simple to implement, showed sensitivity, specificity, positive predictive and negative predictive values of 100, 96.9, 84.6 and 100%, respectively.