SUMMARYThe structure/anti‐fungal relationships of a group of 137 sulphur‐containing compounds were studied by observation of their effects on the mycelial growth of six fungi,Pythium ultimum, Sclerotinia fructigena, Verticillium albo‐atrum, Alternaria solani, Botrytis cinereaandAspergillus niger, and the germination of spores ofBotrytis cinevea.These compounds consist of (a) salts and (b) carboxy‐alkyl and ‐alkenyl esters of both alkyl‐ and aryl‐substituted dithiocarbamic and hydroxydithioformic acids, (c) certain cyclic derivatives of (b), and (d) alkyl and aryl isothiocyanates. Short homologous series were selected to assess the importance of physico‐chemical properties in fungitoxicity, whilst certain aryl‐substituents were used on the basis of their contrasting electronic effects.Derivatives of alkoxydithioformic acids showed higher toxicity than those ofN‐alkyl‐substituted dithiocarbamic acids, except for a series of β‐(N‐alkylthio‐carbamoylthio)acrylic acids. The fungitoxicity of esters of alkyl‐substituted dithiocarbamic or hydroxydithioformic acids showed little change on increasing the size of the alkyl substituent or ester grouping. The fungitoxicity of compounds derived fromN‐aryldithiocarbamic acids varied considerably wit