A 7-year old boy visited the dental hospital complaining that the upper left second deciduous molar had not erupted. Radiographs showed that the crown of this tooth existed near the apex of the upper left first molar. Inversion of the second deciduous molar and second premolar was definite. There were no differences in location, size and shape between the left and right second premolars. A tooth-like object was observed near by the germ of the left second premolar. The impacted molar was examined histopathologically after extraction and consequently it was found that it was a supernumerary tooth. New radiographs were made which suggested that the succedaneous premolar developed normally. The tooth germ of the primolar develops on the lingual side of the deciduous molar and moves in the level of the occlusal plane of the primary molar. Later they are found between the divergent roots and, at the end of the pre-eruptive phase, below the roots of the primary molars. We concluded that the supernumerary tooth disturbed this adjustment.