Previous research has demonstrated that English /r/ is perceptually more dissimilar from Japanese /r/ than English /l/ is for native Japanese (NJ) speakers. It has been proposed by the Speech Learning Model that the more distant an L2 sound (phonetic segment) is from the closest L1 speech sound, the more learnable the L2 sound will be (in: W. Strange (Ed.), Speech Perception and Linguistic Experience: Issues in Cross-language Research, York Press, Timonium, MD, 1995, p. 233). This hypothesis was evaluated in this study by investigating whether NJ speakers will have more success acquiring English /r/ than /l/. A longitudinal study examined the perception (Experiment 1) and production (Experiment 2) of English /l/, /r/, and/w/ by NJ adults and children who were living in the US at the time of testing. The results suggested that there was greater improvement for English /r/ than English /l/ among the NJ children. The NJ children's discrimination of /l/-/r/ and /r/-/w/ was significantly better at the second testing (T2) than 1 year earlier (T1). The NJ children also showed greater improvement from T1 to T2 in producing /r/ than /l/. The results are taken as support for a hypothesis of the Speech Learning Model (in: W. Strange (Ed.), Speech Perception and Linguistic Experience: Issues in Cross-language Research, York Press, Timonium, MD, 1995, p. 233) that degree of perceived phonetic dissimilarity influences L2 learners' success in acquiring L2 phonetic segments. © 2003 Elsevier Ltd. All rights reserved.