This chapter reports preliminary results of an acoustic analysis of fricatives in American English produced by native Japanese-speaking adults and children. The data were from 16 native Japanese (NJ) adults and 16 NJ children as well as 16 native English-speaking (NE) adults and NE children. All 64 participants were tested twice, one year apart to study NJ adults’ and children’s learning of English. Their production of /s/ in the word ‘six’ was acoustically analyzed. Noise duration (absolute and normalized), fricative intensity, and center of gravity (CoG) were measured. For duration, the NJ adults’ /s/ was longer than the NE adults’ /s/ in both absolute duration and normalized duration. The CoG values were greater in the NE adults’ and children’s /s/ than in the NJ adults’ and children’s /s/. Noise amplitude was greater in the NE speakers’ /s/ than in the NJ speakers’ /s/. These results suggest that the NJ speakers’ /s/ was slightly more back than the NE speakers’ /s/ productions and that the NE speakers’ /s/ was more sibilant than the NJ speakers’ /s/. In addition, the noise amplitude increased significantly over the course of one year in the NJ children’s productions.