The present study investigates how listeners of Finnish and Japanese, languages with very similar contrasts in plosive quantity (short vs.long), use language-specific phonetic knowledge of acoustic attributes which covary with closure duration. A fully-crossed perceptual experiment on consonant quantity categorization was conducted with 22 Finnish and 20 Japanese listeners, using natural speech stimuli with systematically manipulated closure durations. Stimuli were created from Finnish and Japanese productions of both long and short plosives. In the naturally produced stimuli, the duration of the vowel preceding the target consonant was affected by language-specific word prosody patterns. The results showed an across-the-board effect of theoriginal quantity of the produced stimulus, with the stimuli created from words with a geminate consonant tending to be perceived as geminate, irrespective of the language of the listeners or talkers, suggesting a strong influence of cues besides closure duration that are shared by the two languages. However, Japanese listeners were more heavily affected by the acoustic cues concomitant to the quantity contrast in their native language, likely due to robust language-specific vowel duration effects in the Japanese productions. Word prosody, besides creating subtle shifts in category boundary for both language groups, created confusions in the listener responses, especially when the language-specific word-level prosodic effect is localized in the vowel preceding the target word, and when the listener is confronted with stimuli from the other language. Thus, some aspects of the quantity contrast are remarkably similar between the two languages, and listeners from one language group are attuned to attributes found in the other language. However, other attributes appear to be language specific, and such durational differences may interfere with categorization in an unfamiliar language.