This study investigated how students' self-reported individual differences predicted second language (L2) spoken discussion task output, an objective behavioral outcome, in the Japanese university English as a Foreign Language (EFL) context. Although numerous psychological theories are used as a rationale for task-based language teaching (TBLT), few studies have investigated the impact of individual differences variables on task performance. To address this gap, a cross-validation procedure was used with students (N = 439) from two different universities. They completed questionnaires to measure willingness to communicate (WTC), speaking self-efficacy (SSE), and perceived communicative competence (PCC). They also engaged in a quasiacademic eight-minute group discussion task (TBLT design). This discussion was recorded and transcribed, with the number of words produced used as an objective measure of L2 task production. In the better fitting mediation structural equation model, the influences of SSE and PCC on spoken L2 task production were fully mediated by WTC (R[superscript 2] = 0.21).