The present study examines the differential use of formal explicit knowledge and intuitive implicit knowledge in a second‐language grammatically judgement task. The hypothesis is that a set of conditions can be established which serve to identify the occasions in which each of these specialized types of knowledge will be used. These conditions are described as task‐related factors (amount of detail required, response time allotted, specific structure tested) and learner‐related factors (level of study in the L2, knowledge of other languages).The subjects for the study were 317 English‐speaking high school students and adults learning. French as a second language French sentences were presented on tape and decisions about the grammatically of each were recorded by subjects on coding sheets.The results indicated that explicit knowledge intervenes for incorrect sentences requiring detailed responses and that knowledge of another language is beneficial for certain conditions. These findings lead to a discussion of the relationship between implicit and explicit linguistic knowledge in second language prof