This study used interval scaling to assess degree of perceived foreign accent in English sentences spoken by native and non-native talkers. Native English listeners gave significantly higher (i.e., more authentic) pronunciation scores to native speakers of English than to Chinese adults who began learning English at an average age of 7.6 years. The results for the “child learners” suggest that a sensitive period for speech learning is reached long before the age of 12 years, as commonly supposed. Adults who had lived in the U.S. for 5 years did not receive higher scores than those who had lived there for only 1 year, suggesting that amount of unaided second-language (L2) experience does not affect adults’ L2 pronunciation beyond an initial rapid stage of learning. Native speakers of Chinese who rated the sentences for foreign accent showed the same pattern of between-group differences as the native English listeners. The more experienced of two groups of Chinese listeners differentiated native and non-native talkers to a significantly greater extent than a less experienced group, even though the subjects in both groups spoke English with equally strong foreign accents. This suggests that tacit knowledge of how L2 sentences “ought” to sound increases more rapidly than the ability to produce those sentences. © 1988, Acoustical Society of America. All rights reserved.