In South Africa, foundation programmes are a well-established alternative access route to tertiary science study for educationally disadvantaged students. Student access to, and performance in, one such foundation programme has been researched by the authors seeking opportunities to improve student retention. The biology module in particular has been recognised to place students at risk of failing the foundation programme, thereby reducing throughput into mainstream science programmes. This study uses decision tree analysis to provide a detailed description of foundation biology student performance so that points of weakness and opportunities for remedial action may be pinpointed. While students' alternative-entry selection scores have previously been found to most effectively account for performance in the programme as a whole, no similar positive relationship was identified for any subgroup of students in the foundation biology module. Conversely, academic language proficiency in the medium of instruction (English), formerly found to play no role in overall student performance, was revealed as primary in explaining achievement in foundation biology, most adversely affecting students rendered particularly vulnerable by an additional academic and/or socio-economic disadvantage. A pass in the stand-alone foundation academic literacy module did not necessarily correspond to a pass in biology. Compromised by educational disadvantage, compounded by a mismatch in programme selection criteria and inadequate academic literacy support, discipline-specific, fundamental literacy development in the biology curriculum is proposed to enable students towards epistemic access in the module. Pending this intervention, formal access to mainstream study is unlikely for the foundation students most at risk of failure.