Abstract: The problem. The purpose of this study was to investigate whether there would be a significant difference between an explicit-inductive/cooperative instruction and an explicit-deductive/individualistic instruction for the acquisition of English relative clauses in Korean university-level EFL learners. Method. This study used a mixed method design of data collection and analysis. The subject pool consisted of 90 undergraduate EFL students at Cheongju University, Korea. Two intact classes of 45 students each were randomly assigned to one of the treatments. Both groups received instruction about English relative clauses using the explicit-inductive/cooperative approach (experimental group), or the explicit-deductive/individualistic approach (control group). Students were administered a pretest, a posttest, and a delayed posttest with two sections: a Sentence Combining Test (SCT) and a Grammaticality Judgment Test (GJT). T-tests of dependent and independent samples were carried out to test for significant within groups and across group differences. To provide more reliability and validity for the results of quantitative data analysis, the results of the qualitative data analysis were triangulated with the quantitative results. Results. Both groups significantly increased their overall learning outcomes from the pretest to the posttest, but the experimental group scored significantly higher than the control group. Therefore, the explicit-inductive/cooperative instruction was more effective than the explicit-deductive/individualistic instruction. Also, both groups consistently scored higher on the SCT than on the GJT, but the experimental group scored significantly higher than the control group on both the SCT and the GJT. The rank order of mastery over four types of English relative clauses was OS > OO > SS > SO, and the rank order of mastery over four types of common errors of English relative clauses was pronoun retention > incorrect relative-marker morphology > inappropriate relative-marker omission > nonadjacency. EFL learners reported that they preferred cooperative learning over the explicit-inductive, discovery grammar activities. However, both the explicit-inductive, discovery grammar activities and cooperative learning were positively rated. Six key themes were found regarding EFL learners' perceptions of explicit-inductive, discovery grammar activities: (1) learner-centeredness; (2) complete understanding of the concepts; (3) positive attitudes towards L2 grammar learning; (4) creative thinking; (5) neutrality toward the explicit-inductive, discovery grammar activities; and (6) preference for the traditional explicit-deductive grammar activities. Seven key themes were found regarding their perceptions of cooperative learning: (1) Communicative Language Teaching (CLT) through more communication and interaction; (2) interdependence through more opportunities to answer questions; (3) more comfortable, safer, and less competitive learning environment; (4) active and independent learners; (5) learner-centeredness, (6) neutrality toward cooperative learning; and (7) preference for individualistic learning.