It has been observed that there is a strong association between the inherent (lexical) aspect of verbs and the acquisition of tense-aspect morphology (the aspect hypothesis; Andersen & Shirai, 1994). To investigate why such an association is observed, this study examined the influence of inherent aspect and learners' first language (L1) on the acquisition of Japanese imperfective aspect by using two tasks--an acceptability judgment test and an oral picture description task--with two groups of second language learners of Japanese: 26 native speakers (NSs) of English, which has the obligatory progressive, and 35 NSs of languages that have no obligatory progressive marking (German and Slavic languages). The results from the acceptability judgment test support the aspect hypothesis in that, regardless of L1, the imperfective marker "-te i-ru" was strongly associated with activity verbs for lower proficiency learners. However, the results from the oral task did not support the prediction, in that lower proficiency L1 nonprogressive learners did not show any such preference. The results suggest that L1 plays a role in the formation of the acquisition pattern predicted by the aspect hypothesis, but that given the complex interaction with task types and proficiency, L1 transfer cannot be the sole reason for the predicted association in the acquisition of Japanese "-te i-(ru)." It is argued that multiple factors are at work when learners create the form-meaning associations predicted by the aspect hypothesis.