Examined the development of sensitivity to 2 musical relations significant in Western tonal music, the semitone and diatonic structure, in 2 experiments with 30 preschoolers (aged 4.2-5.9 yrs) and 40 infants (aged 9.1-11 mo). Ss were tested for their detection of a semitone change in any position of a 5-note melody. Two standard melodies were used, one composed of diatonic tones only and the other containing a nondiatonic tone. In Exp I, preschoolers were superior in detecting the semitone change in the diatonic context compared with the nondiatonic context. In Exp II, infants detected the semitone change in all positions, but their performance was not influenced by diatonic context. It is concluded that infants and children can discriminate a semitone in a musical context and that the priority of diatonic structure emerges by 4-6 yrs of age.