One of the great mysteries of language development is how children acquire language so efficiently while adults are never able to reach the same level of proficiency. Adding to this mystery is that child learners rarely receive negative evidence regarding the nature of the grammatical structure of their language, but adults are more likely to receive and use such evidence (in classes, corrections, etc.) (Baker, 1979). The present study tests the role of negative evidence in adult language learners, who were exposed to an artificial grammar characterized by round vowel harmony, a phonological process whereby vowels agree in the feature round. Participants were exposed to either positive evidence only (Positive Evidence Condition), or both negative and positive evidence (Positive Evidence Condition). In two experiments, participants in the Positive Evidence Condition outperformed participants in the Negative Evidence Condition, specifically for test items tat measured extension of learned items to novel items. These results suggest that negative evidence may hinder adult grammatical rule learning.