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Considering the future of the application of a dual explicit-implicit learning system to the L2 theory and research, Ellis (2006) argues that further investigation of the distinction is useful for modeling, understanding, and measuring second language proficiency. This study explored the differential accessibility of EFL learners' explicit and implicit grammatical knowledge to their language proficiency. The participants were 160 EFL graduate and undergraduate students at Shahrekord University (Iran). A test battery including a timed grammaticality judgment test (GJT), an untimed GJT, and a TOEFL was used to gather the data. A set of correlation coefficients was computed to explore the contributions of implicit and explicit grammatical knowledge to the TOEFL and its sub-components. The results showed that there was no statistically significant correlation between the EFL learners' implicit grammatical knowledge and their TOEFL (sub-components) scores, but there was a strong relationship between the EFL learners' explicit grammatical knowledge and their general proficiency. A medium relationship also existed between the explicit knowledge and the TOEFL sub-components. Then, a Standard Multiple Regression demonstrated that explicit knowledge better predicted the EFL learners' general L2 proficiency. The results suggest that learning explicit grammatical knowledge is necessary in EFL contexts and needs much more consideration when the primary focus is on the cognitive academic language proficiency or skills.