Recent studies in productive phonology among immigrants have supported the case for the existence of a critical or sensitive period in language acquisition. The present study proposes that the case for a sensitive period would be further strengthened by the discovery of comparable age effects for other linguistic abilities such as listening comprehension. It was hypothesized that the most powerful predictor of comprehension would be the age at which the individual had begun using English and that the amount of practice with second language and other attitudinal and usage variables would show only indirect relationships with comprehension skill. Sixty male Italian immigrants listened to taped English sentences masked with background noise at four signal-to-noise ratios. Comprehension scores were correlated with age of beginning English but were not predicted by number of years in the United States or by other variables investigated. Scores of those who arrived before early adolescence resembled those of native speakers, while those subjects who began English toward the end of adolescence showed a marked comprehension deficit. These results supported the hypothesis that a sensitive period exists for the acquisition of a second language. The discussion explores the sensitive period as a developmental concept and treats two themes that occur frequently in the acquisition literature: the importance of motivation and the role of input. (Author/AMH)