This study examined the influence of the L1 phonetic system on the acquisition of Swedish quantity distinctions. The aim was to test the hypothesis that difficulty in acquiring an L2 contrastive category is related to the role in the L1 of the phonetic feature upon which the L2 category is based. Twenty native speakers each of American English, Latin American Spanish and Estonian participated. The phonologies of the subjects' L1 display three degrees of overall prominence of the duration feature. The subjects, who had all lived in Sweden for at least 10 years, were given a production and perception test to assess their mastery of Swedish quantity. The Estonian subjects performed much like Swedish controls. However, some native English, and even more Spanish subjects, differed from native speakers of Swedish. A follow-up experiment compared matched groups of native English and Spanish subjects. It showed that experienced and inexperienced groups of native English and Spanish adults were equally successful in learning to produce and perceive Swedish quantity distinctions, but that the native English subjects more closely matched native Swedish speakers than the native Spanish subjects did. Taken together, our results support the feature hypothesis in that these subjects' success in learning the Swedish quantity contrast seems to be related to the role of the duration feature in the L1. © 2002 Elsevier Science Ltd. All rights reserved.