Previous research has suggested that child but not adult immigrants to the United States and Canada make regular progress learning English as their length of residence (LOR) increases. If children and adults received the same kind of second language (L2) input, such evidence would support the existence of a critical period for L2 acquisition. The present study compared groups of Chinese adults living in the United States who differed in LOR in order to assess the role of input in adults' naturalistic acquisition of an L2. We assessed the Chinese participants' identification of word-final English consonants (experiment 1), their scores on a 144-item grammaticality judgment test (experiment 2), and their scores on a 45-item listening comprehension 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 with relatively 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 each instance. The results suggested that the lack of an effect of LOR in some previous studies may have been due to sampling error. It appears that adults' performance in an L2 will improve measurably over time, but only if they receive a substantial amount of native speaker input.