The present study examines the acquisition of the morphological forms which mark aspectual meanings by Japanese-speaking learners of English and English-speaking learners of Japanese with a range of proficiency levels. To investigate the L2 learners' knowledge of aspectual interpretation, an acceptability judgment test was conducted. Given that the L2 acquisition of English aspect involves learning of uninterpretable features whereas the L2 acquisition of Japanese aspect does not, the existing hypothesis that L2 learners are unable to learn uninterpretable features which are not instantiated in their L1 would predict that the L2 acquisition of English aspect is impossible and that of Japanese aspect is possible. The results show that the learners of L2 English with the advanced proficiency level assign correct meanings to the morphological forms concerned. In contrast, the learners of L2 Japanese cannot assign all the possible meanings to the morphological forms. Therefore, it can be suggested that interpretability of formal features does not necessarily explain the success or the failure of second language acquisition.