Fungicidal and algicidal substances are obtained by reacting acidic R₁R₂N-H compounds with C1-S-CX₃ (X = halogen) to N-tri= halogenmethylthio compounds (R₁R₂N-S-CX₃). The toxophoric group preferred is the S-CCl₂F group. The mechanism of action of the trihalogenmethylthio compounds is based on the N-S bond's capacity to open and thus to react with nucleophilic groups, e.g., SH groups. Optimum effectiveness is achieved with compounds whose N-S bond has a medium stability. A measure characterizing the stability is the rate of hydrolysis at a pH of, say 8. Hydrolysis yields cleavage products that are void of microbicidal effectiveness and no longer ecotoxic. Taking account of these facts, it is possible to select, from the wide range of trihalogenmethylthio compounds already available, microbicides with such chemico-physical and toxicological properties as are suited to defined uses, e.g., as fungicides and algicides in plastics, paint films, wood preservatives, and antifouling coatings. The tailoring of microbicides is a further interesting possibility.