Vowel discrimination in 8-wk-old infants was investigated using a non-nutritive, high-amplitude sucking measure in a habituation-dishabituation paradigm. Infants were presented pairs of vowel stimuli differing in equal logarithmic steps along the /i/ to /I/ continuum. Stimulus pairs of equal acoustic differences were selected to be either within or between the adult phonetic categories. Infants were divided into 3 experimental stimulus shift conditions; within category, between category and control. Using a scoring system to assess perinatal events, half the infants in each condition were categorized as being at "higher risk" for developmental problems. As evidenced by changes in high-amplitude sucking following stimulus shift, significant differences (P < .05) were found between the control and both within and between phonetic category conditions. No effects due to risk score population differences proved significant. Eight week old infants discriminate the vowels /i/ and /I/ in a continuous as opposed to a categorical manner. Some differences were noted in response patterns between high- and low-risk score infants.