Numerous findings suggest that non-native speech perception undergoes dramatic changes before the infant' s first birthday. Yet the nature and cause of these changes remain uncertain. We evaluated the predictions of several theoretical accounts of developmental change in infants' perception of non-native consonant contrasts. Experiment 1 assessed English-learning infants' discrimination of three isiZulu distinctions that American adults had categorized and discriminated quite differently, consistent with the Perceptual Assimilation Model (PAM: Best, 1995;Best et al., 1988). All involved a distinction employing a single articulatory organ, in this case the larynx. Consistent with all theoretical accounts, 6-8 month olds discriminated all contrasts. However, 10-12 month olds performed more poorly on each, consistent with the Articulatory-Organ-matching hypothesis (AO) derived from PAM and Articulatory Phonology (Studdert-Kennedy & Goldstein, 2003), specifically that older infants should show a decline for non-native distinctions involving a single articulatory organ. However , the results may also be open to other interpretations. The converse AO hypothesis, that non-native "between"-organ distinctions will remain highly discriminable to older infants, was tested in Experiment 2, using a non-native Tigrinya distinction involving lips versus tongue tip. Both ages discriminated this between-organ contrast well, further supporting the AO hypothesis. Implications for theoretical accounts of infant speech perception are discussed. (Contains 2 tables, 2 figures and 8 footnotes.)