The aim of the action research reported here was to examine the differential effects of the 'writers' workshop' approach on the L2 (English) writing skills of upper-primary students with varying writing abilities. The participants were 31 fifth-grade students (17 boys and 14 girls) aged 10-11, who followed L2 English writing instruction based on the 'writers' workshop' approach at a private primary school in Beirut, Lebanon. The writers' workshops were conducted twice a week over a period of eight consecutive weeks. An opinion essay served as a pre-test and a post-test in order to gauge writing improvement. Findings revealed statistically significant improvement in students' writing-related outcomes across all writing ability groups. However, the writers' workshops intervention was found to result in a differential group effect: generally, above-average writers benefitted more than less-proficient writers. This was particularly clear in relation to the content component of their essay writing.