This study examined pronunciation proficiency in both the first (Korean) and second (English) languages of bilinguals. The participants were adult immigrants whose age of arrival in the USA ranged from 1–23 years. English and Korean sentences were rated by native listeners to obtain measures of pronunciation proficiency. English pronunciation of participants with ages of arrival of 1–5 years was close to monolinguals, heavier accents were noted as ages of arrival increased from 6 to 23 years. Korean pronunciation of participants with ages of arrival of 1–7 years was distinctly accented, while those with ages of arrival of 12–23 years were rated the same as monolinguals. Participants with ages of arrival of 1–9 years pronounced English better than Korean, whereas the reverse was true for ages of arrival of 12–23 years. Overall, the results were more consistent with the view that deviations from native pronunciation result from interactions between the languages of bilinguals rather than with the view of a maturationally defined critical period for language learning.