Japanese allows null subjects and null objects, while English allows neither. This difference implies that the acquisition of obligatory objects is as difficult as the acquisition of obligatory subjects for Japanese learners of English. Contrary to this prediction, data in this study suggest that obligatory objects are far more difficult than obligatory subjects for Japanese learners of English (JLEs). With the same kind of data from Chinese learners of English, Yuan (1997) suggests that this asymmetry is attributable to the availability of triggering data in input : the availability of morphological evidence of subject-verb agreement triggers the acquisition of obligatory subjects in Chinese learners' L2 grammar, while there are no such triggering data for the acquisition of obligatory objects. We argue that although Yuan's explanation, with his assumption that learners' use of morphemes does not reflect syntactic knowledge, cannot be refuted, it is not supported by experimental data. Davies (1996), for example, showed that the acquisition of morphological subject-verb agreement and the acquisition of obligatory subjects are independent of each other. We suggest three factors for the asymmetry. One is the difference in the consistency in L2 input. Namely, subject noun phrases are consistently phonetically realized in every sentence in English, while object noun phrases are not; i.e., sentences with intransitive verbs lack object noun phrases. With this kind of input, JLEs may believe null objects to be allowed in English. In addition, the verbs that can be used as both transitive and intransitive may lead JLEs to take a wrong track. Moreover, the lack of morphological markings for transitivity on English verbs may make it difficult for JLEs to learn the syntactic properties of each verb.