The mechanical properties of the seed shells of the African mongongo nut, Schinziophyton rautenenii (Euphorbiaceae), were measured by compressive C-ring tests in an air-dry condition and also after soaking in distilled water. Young’s modulus was found to be about 5 GPa and the fracture strength was 40-50 MPa, for both conditions. However, fracture toughness was affected significantly by moisture content. The critical stress intensity factor, K-IC, of air-dried specimens was 27% greater and the work of fracture, R, 69% greater than those of wet specimens. This difference corresponded well with microscopic observations of the complexity of the fracture surface. Viewed either by scanning electron microscopy or confocal microscopy, cracks in the wet shell deviated neatly around individual fibres, while cracks in air-dried shells either crossed individual fibres or ran obliquely across the outer layers of the secondary cell wall leaving a feathered appearance. It is proposed that the increase in toughness of shells which would be obtained from air-drying may help protect embryonic seed tissues from predation by larger animals (e.g, vertebrates such as rodents) after abcission from the parent plant.