In Japanese, vowel duration can distinguish the meaning of words. In order for infants to learn this phonemic contrast using simple distributional analyses, there should be reliable differences in the duration of short and long vowels, and the frequency distribution of vowels must make these differences salient enough in the input. In this study, we evaluate these requirements of phonemic learning by analyzing the duration of vowels from over 11 hours of Japanese infant-directed speech. We found that long vowels are substantially longer than short vowels in the input directed to infants, for each of the five oral vowels. However, we also found that learning phonemic length from the overall distribution of vowel duration is not going to be easy for a simple distributional learner, because of the large base-rate effect (i.e., 94% of vowels are short), and because of the many factors that influence vowel duration (e.g., intonational phrase boundaries, word boundaries, and vowel height). )