This paper discusses the reality and relevance of the prosodic unit 'mora' in Japanese in a cross-linguistic perspective. The main lines of evidence to be discussed are taken from the following three linguistic phenomena: (a) word formation, (b) speech errors, and (c) stuttering. Careful analyses of these phenomena in Japanese reveal that the mora plays an important role as a unit of phonological length, i.e. as a unit for measuring phonological distance in various types of linguistic phenomena. This finding contrasts with the situation in English where essentially the same role is played by the syllable.