Acquiring L2 vowel quantity can be difficult for native speakers of languages like English where vowel duration cues stress. This study tested whether English learners of Czech would categorize short and long vowels in a stressed or in an unstressed syllable differently than native listeners. The role of L2 experience was also explored. Results showed that the native and non-native listeners did not differ in category boundary locations in either syllable, although non-native perception was less categorical in the unstressed syllable. No effect of experience was found. It is concluded that the L2 learners redefined the value of vowel duration as a cue.