Two experiments were conducted, on Japanese and on Arabic, to explore the extent to which vowels adjacent to longer and shorter apical consonants would compensate for the consonant durations. In the Arabic experiment, little evidence of timing compensation was found, whereas Japanese vowels on both sides of a consonant varied inversely with the consonant, and the consonant constriction duration varied inversely with the inherent vowel durations. The result is that the duration of the two-syllable test words scarcely changed at all. These results support the notion that Japanese morae have constant durations although they undermine the definition of the mora as a simple CV unit. These findings suggest that rhythmic structures in speech need not be thought of as constructed by the perceiver; they may also be present in the acoustic signal itself. More generally, our results imply the existence of language-specific timing rules that cannot be captured in the segmental terms insisted upon by current linguistic phonetic theory.