This study examined production and perception of the contrast between /t/-/d/ by subjects differing in native language and age. Acoustic analysis revealed that native speakers of English realized word-initial /t/ with significantly longer VOT values (approximately 85 ms) than native speakers of Spanish (approximately 19 ms). Native English and Spanish adults realized /t/ with VOT values that were non-significantly longer than those of 9-year-olds of the same native language background. Native English adults prevoiced /d/ more often than English children, but Spanish adults and children realized /d/ with lead VOT values in nearly every instance. In a labelling task, the native English speakers showed steeper identification functions, and category boundaries at significantly longer VOT values, than age-matched native Spanish subjects. The boundaries of both the native English and Spanish adults occurred at significantly longer VOT values than those of children who spoke the same language. Three possible explanations are offered for the effect of age on stop voicing judgments: The auditory processing of acoustic parameters associated with stops changes with age listeners require an increasingly long VOT interval to perceive /t/ as they get older because they produce /t/ with somewhat longer VOT values; listeners 'attune' their perception to optimally match the stops they hear. © 1986 S. Karger AG, Basel.