Determination of the proximate cause of sexual dimorphism remains difficult, especially when trying to discriminate between sexual selection and ecological causation. A clear rejection of one of these hypotheses would advance the direction of future investigations, especially for cryptic and/or subterranean species. Sexual dimorphism in head size, but not body size, is confirmed for Schistometopum thomense, a fossorial caecilian from the island of São Tomé, in the Gulf of Guinea. However, a quantitative and qualitative study of diet reveals no significant difference between males and females sampled from three sites on the island. Females appear to take larger and heavier prey (principally earthworms) than males, despite having a significantly smaller head size. We tentatively reject the ecological causation hypothesis, and discuss several testable hypotheses for evidence of sexual selection.