A problem facing investigations of implicit and explicit learning is the lack of valid measures of second language implicit and explicit knowledge. This paper attempts to establish operational definitions of these two constructs and reports a psychometric study of a battery of tests designed to provide relatively independent measures of them. These tests were (a) an oral imitation test involving grammatical and ungrammatical sentences, (b) an oral narration test, (c) a timed grammaticality judgment test (GJT), (d) an untimed GJT with the same content, and (e) a metalinguistic knowledge test. Tests (a), (b), and (c) were designed as measures of implicit knowledge, and tests (d) and (e) were designed as measures of explicit knowledge. All of the tests examined 17 English grammatical structures. A principal component factor analysis produced two clear factors. This analysis showed that the scores from tests (a), (b), and (c) loaded on Factor 1, whereas the scores from ungrammatical sentences in test (d) and total scores from test (e) loaded on Factor 2. These two factors are interpreted as corresponding to implicit and explicit knowledge, respectively. A number of secondary analyses to support this interpretation of the construct validity of the tests are also reported. [This research was funded by the Royal Society of Arts of New Zealand.]