Elemental sulphur (S0) is man's oldest fungicide. In biological systems it is formed by certain specialized prokaryotes but the element has rarely been found in eukaryotes. The recent discovery that certain plant species from diverse families produce S0 as a localized component of active defence to vascular pathogens, and that S0 is constitutive in some crucifers, led to this review. Because of the age and relative inaccessibility of some of the past literature and the inconsistency in the methods used, the spectrum of activity and the toxicity of S0 are reassessed here. Interpretation of bioassays of this and other hydrophobic compounds are offered. Also, brief coverage is given to the history of S0 use and its suggested mode(s) of action. The element's possible role in defence and the form, location and levels in planta are considered. Sulphur is one of many S-containing defence-related compounds and it is ironic that sulphur deficiency has recently become a widespread nutrient disorder in crops, largely due to restrictions on fossil fuel burning. The problem is being addressed by sulphur application, but the future manipulation of genes for sulphate uptake and sulphur biosynthesis are likely goals.