Previous research has suggested that child but not adult immigrants to the United States andCanada make regular progress learning English as their length of residence (LOR) increases. Ifchildren and adults received the same kind of second language (L2) input, such evidence wouldsupport the existence of a critical period for L2 acquisition. The present study compared groupsof Chinese adults living in the United States who differed in LOR in order to assess the role ofinput in adults' naturalistic acquisition of an L2. We assessed the Chineseparticipants' identification of word-final English consonants (experiment 1), their scores ona 144-item grammaticality judgment test (experiment 2), and their scores on a 45-item listeningcomprehension test (experiment 3). The Chinese participants were assigned to one of four groups(n = 15 each) based on LOR in the United States and their primary occupation(students vs. nonstudents). Significantly higher scores were obtained for the students withrelatively long LORs than for the students with relatively short LORs in all three experiments.However, the difference between the nonstudents differing in LOR was nonsignificant in eachinstance. The results suggested that the lack of an effect of LOR in some previous studies mayhave been due to sampling error. It appears that adults' performance in an L2 will improvemeasurably over time, but only if they receive a substantial amount of native speaker input.