The paper reports on production of English lexical stress by fluent Japanese speakers of English. The Japanese speakers showed similar patterns to native English speakers for vowel duration and F0 of stressed and unstressed vowels. However, the Japanese speakers' control of vowel quality was different from English speakers. The English speakers varied their F1 and F2 values depending on whether vowels were stressed or not; unstressed vowels were centralized. However, vowel stress had no effect on the vowel quality of the Japanese speakers' utterances. They expressed English accent by duration, F0 and intensity. Therefore, Japanese speakers appear to be more sensitive to duration and F0 in English, which are phonologically important features in Japanese, but are not very sensitive to vowel quality. These results imply that in the process of L2 acquisition, some acoustic features are easier to acquire than others, and for Japanese speakers it is difficult to acquire L2 phonemes.