A comparison was made between the effects of 15 and 60 seconds of etching with a 37 percent phosphoric acid solution on enamel surfaces from deciduous and young and old permanent teeth. Resin impressions of fifteen pairs of enamel surfaces from each group of teeth were studied in the scanning electron microscope. The results were based on in-pair evaluations. A scale from 0 to 3 was used to evaluate the degree of surface irregularity. For deciduous teeth, no difference was found in effect between the etching periods studied. For young permanent teeth, 15 seconds of etching created more retentive conditions than 60 seconds. For old permanent teeth, the reverse was found. The most retentive conditions were found for the deciduous teeth, regardless of etching time.