The aim of the paper is to study the perception of Estonian vowel categories by L2 learners of Estonian whose L1 is Russian. Estonian vowel system includes nine vowels whereas Russian has six. Five of Estonian vowels have counterparts in Russian: /i/, /e/, /u/, /o/ and /a/, the new vowel categories for L2 speakers are /ü/, /ö/, /ä/, and partly /õ/. For the perceptual experiments four-formant vowel stimuli were synthesized including nine Estonian prototype vowels and the intermediate steps between prototypes; the stimuli set covered 14 vowel category boundaries. The experiments involving native Estonian and non-native (Russian as L1) subjects showed that (1) Estonian vowels /i/, /e/, /u/ and /o/ assimilate well with their Russian counterparts; (2) Estonian /a/ and /ä/ assimilate with the allophones of Russian /a/; (3) Estonian /ü/, /ö/ and /õ/ assimilate partly with Russian /?/; due to the close phonetic distance L2 subjects' ability to discriminate these categories is poor.