Although a growing body of research in writing has suggested that females outperform males in many aspects of writing, our understanding of gender differences is still limited. The present study aimed to examine the potential sources of gender differences in writing fluency and text quality across Arabic as a first language (L1) and English as a foreign language (FL). 77 undergraduate Omani students produced two argumentative texts, one in Arabic and one in English. Their English proficiency was assessed using the Oxford Placement Test (OPT). Their writing was recorded and analysed via keystroke logging. The study found that females outperformed males in terms of writing fluency and text quality. Findings also indicated that females' superiority in writing fluency and text quality is a consequence of their superiority in English proficiency. Furthermore, findings suggested that writing fluency in English is an important explanatory variable that accounted for females' superiority in text quality. Findings are discussed in light of process-oriented writing research and implications for writing research and teaching are suggested.