Whereas most research in SLA assumes the relationship between the first language (L1) and the second language (L2) to be unidirectional, this study investigates the possibility of a bidirectional relationship. We examine the domain of manner of motion, in which monolingual Japanese and English speakers differ both in speech and gesture. Parallel influences of the L1 on the L2 and the L2 on the L1 were found in production from native Japanese speakers with intermediate knowledge of English. These effects, which were strongest in gesture patterns, demonstrate that (a) bidirectional interaction between languages in the multilingual mind can occur even with intermediate proficiency in the L2 and (b) gesture analyses can offer insights on interactions between languages beyond those observed through analyses of speech alone.