The tangential ground surfaces of the occlusal enamel in maxillary premolar teeth, extracted from young orthodontic patients, were observed by scanning electron microscopy after EDTA etching. At the junctions of the prism orientation disorders, various abnormal-shaped prisms containing many circular shapes were scattered in a similar way to our recent study on the lingual enamel of maxillary incisor teeth. Abnormal-shaped prisms were also observed in the areae adjacent to the clear spiral region of prisms in the gnarled enamel, however, they were not found in the spirally arranged prisms which agreed to our previous observations. On the other hand, one or a few abnormal-shaped prisms were occasionally present in the regions showing an ordinal orientation of the prisms. From our present observations and a large number of previous studies on abnormal-shaped prisms, such prisms commonly appear in human enamel although their average to the entire enamel prisms in a tooth is low. Thus, we suggest that abnormal-shaped prisms, especially circular structures as a primitive type of enamel prisms, are always found to be present.