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According to the identity group interpretation of the quantity opposition in Finnish, long vowels are perceived as two successive short vowels of the same spectral quality. Some recent studies, however, challenge this general view. To investigate this, 16 listeners were first asked to categorize four sets of 19 synthesized stimuli, each set representing the Finnish vowel continuum /y/–/i/ at one of the following stimulus durations: 50 ms, 100 ms, 250 ms, and 500 ms, which cover the reported durational variations of short and long Finnish vowels. The stimuli on the /y/–/i/ continuum varied for the second formant (F2) in steps of 30 mel. Large individual variation was found in the categorization, but the category boundary F2 value and the boundary width were independent of duration in the group level, suggesting that quantity does not affect the category formation between /y/ and /i/. Normalized reaction times showed that the categorization was most difficult at 100 ms, that is, a duration that falls between a typical short and long Finnish vowel. Following the categorization task, in order to find the prototypical /i/, the same listeners were asked to evaluate the goodness of those vowels they had individually identified as /i/. The goodness rating scores and F2 frequencies of the /i/ prototypes thus found were essentially the same at all durations, suggesting that phoneme prototypes are not demonstrably dependent on the phonological quantity opposition. In conclusion, the results of this study are in accordance with the identity group interpretation of Finnish quantity opposition.