This study examined Japanese speakers' learning of American English during their first years of immersion in the United States (U.S.). Native Japanese-speaking (NJ) children (n=16) and adults (n=16) were tested on two occasions, averaging 0.5 (T1) and 1.6 years (T2) after arrival in the U.S. Age-matched groups of native English-speaking children (n=16) and adults (n=16) also participated. The NJ adults' scores for segmental perception and production were higher than the NJ children's at T1. The NJ children's foreign accent scores and pronunciation of English fricatives improved significantly between T1 and T2, whereas the NJ adults' scores did not. These findings suggest that adults may have an advantage over children initially but that children may improve in production more quickly than adults when immersed in an L2-speaking environment. (Contains 5 tables, 1 figure and 7 footnotes.)