Drawing upon Vygotsky's Sociocultural Theory (SCT), this study aimed at investigating the effect of two writing modes, namely, peer-mediated/collaborative vs. individual writing on measures of fluency, accuracy, and complexity of female EFL learners' writing. Based on an in-house placement test and the First Certificate in English writing paper, a total of 108 intermediate (N = 54) and advanced (N = 54) participants out of a pool of 150 EFL learners were randomly selected and assigned equally to collaborative (N = 18 pairs in each proficiency level, that is, intermediate and advanced) and individual writing classes (N = 18 participants in each proficiency level). All the participants wrote six argumentative compositions either individually or collaboratively in six subsequent sessions. Data on detailed discourse analytic measures of fluency, accuracy, and complexity obtained from the written compositions were submitted to a series of a two-way analysis of variance statistics. The findings revealed that, across both proficiency levels, collaborative groups outperformed the individual groups in terms of fluency and accuracy but not in complexity. The results are discussed in relation to effective writing instruction through peer mediation as inspired by Vygotsky's SCT of learning. In line with this theory, it is proposed that learning is a social activity and learners' knowledge development depends on the interaction with others. Therefore, involving the learners in collaborative or group writing may be considered an effective instructional technique in the writing class.