Japanese infant-directed speech (IDS) and adult-directed speech (ADS) were compared on their segmental distributions and consonant-vowel association patterns. Consistent with findings in other languages, a higher ratio of segments that are generally produced early was found in IDS compared to ADS: more labial consonants and low-central vowels, but fewer fricatives. Consonant-vowel associations also favored the early produced labial-central, coronal-front, coronal-central, and dorsal-back patterns. On the other hand, clear language-specific patterns included a higher frequency of dorsals, affricates, geminates, and moraic nasals in IDS. These segments are frequent in adult Japanese, but not in the early productions or the IDS of other studied languages. In combination with previous results, the current study suggests that both fine-tuning (an increased use of early produced segments) and highlighting (an increased use of language-specifically relevant segments) might modify IDS on the segmental level.