It is considered that there are two components of grammatical knowledge: implicit knowledge and explicit knowledge. Ever since Krashen (1981) distinguished acquired knowledge (i.e. implicit knowledge) from learnt knowledge (i.e. explicit knowledge), to what extent learners can command the two kinds of knowledge has been one of the main focuses of second language acquisition research. The present study has its focus on Japanese junior high school students' grammatical knowledge of English. A total of 466 first and third grade students answered two kinds of grammaticality judgement tests: test with hints and test without hints. The results revealed that (a) most students have acquired both of the two knowledge or explicit knowledge only, and only a few have developed implicit knowledge alone, (b) the pattern of grammar acquisition differs among grammatical items, (c) though the third grade students have developed more grammatical knowledge than the first grade students, the patterns of grammar acquisition differ between the groups.
