This study investigated the effects of synchronous and asynchronous CMC (computer-mediated communication)on the development of linguistic features of learners' speech in Japanese. Using learners from fourth-semester Japanese classes, the following research questions were examined: (a) Does CMC have positive effects on the development of oral proficiency among learners of Japanese? and (b) Are there any relationships between language use in CMC and the development of oral skills? The participants were divided into three groups: two CMC groups (synchronous and asynchronous) and one face-to-face group. They engaged in weekly discussions over the course of a semester, and the gain scores in terms of language output, linguistic complexity, accuracy, and cohesive devices between the pre-test and the post-test were compared. In addition, the relationship between language use in both types of CMC and the gain scores on the oral tests was examined, using Pearson correlation coefficients. The present study did not find any significant differences among gains in the linguistic indices. However, it found that CMC language use was correlated positively with the gain scores in several indices, and a relationship between CMC language use and the development of oral proficiency was observed. (Contains 1 figure, 8 tables and 4 notes.)