This study is to analyse the development of tone discrimination in infancy. Method Subjects. 49 infants of four, five and six years. In details, see fig. 1. Experimental stimuli (test and items). Musical Aptitude of Infancy Tokyo: Nihon-bunkakagaku-sha 1969. Test items are loudness, rhythm, pitch, timbre and chord discrimination. Procedure. The subjects were instructed to listen to two different stimuli and judge whether the same tone or different, or, which tone is loud or high. The score was the number of correct answers. The subjects were inquired whether they have the musical environment at home and the experience or not. And, the subjects were classified four group, A: have musical environment and experience, B: have musical environment and no experience, C: have no environment and experience and D: have no musical environment and no experience. Results On five discrimnations, there was the score reversed slightly, but the development from four years to six years was recognized. The difference of the scores among three groups, A, Band C, was analysed, there was not the consistent tendency. The results of D-group showed low level as compared with other groups in most cases. There was the relationship between the musical ability and the musical environment and experience, particularly in rhythm and pitch discrimination. The ability of the discrimination whether the stimuli are different or not, I think, is related to the concentration of attention or the memory.