Observations were made on the variation of molar teeth in mice, Mus musculus s.l., collected from international voyage cargo boats after cyanogas fumigation at Yokohama and Kawasaki Harbour. The defect of the third molar tooth on either upper or lower mandible was detected in 45 of 178 adult mice, at an unusually high rate of 25.2 percent. No defect of the third molar tooth was observed at examination of 50 albino laboratory mice of the European origin.
