A two-alternative forced-choice categorization experiment (2AFC) tested whether the type of tone (static high vs. dynamic fall) affected the perception of the length of a stressed initial syllable in Finnish, when the participants had to categorize it as “short” or “long”. In addition to the main effects of the duration of the first and second syllables, the results showed a significant main effect of tone that was qualified by an interaction with the duration of the first syllable nuclei. More precisely, the participants were ceteris paribus more likely to categorize the vowel of the first syllable as “long” in the dynamic fall condition than the high tone condition. The results showed that, alongside with duration, also the tonal structure is used as a strong perceptual cue for the quantity opposition in Finnish.