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SUMMARY Effects of treatment with phenylthiosemicarbazide (PTS) and its 4′‐chloro‐derivative (4′‐chloro‐PTS) on Fusarium wilt of pea and tomato plants were investigated. Depending on pH and availability of oxygen, PTS and 4′‐chloro‐PTS are converted to their corresponding phenylazothioformamides and phenylazothioformamide‐S‐oxides, which are the actual fungitoxic compounds. PTS and 4′‐chloro‐PTS were shown to inhibit growth of Fusarium oxysporum f. pisi and F. oxysporum f. lycopersici in liquid media as well as on agar plates at concentrations of 50–100 mg/1. Inhibition was greater at pH 7 than at pH 5. When administered to pea and tomato plants, both compounds caused severe phytotoxic effects, especially at temperatures favouring Fusarium wilt, thus almost entirely obscuring any protective activity against the diseases. All compounds were strongly adsorbed to loam, but readily released from sand. Neither in pea nor in tomato plants were PTS and 4′‐chloro‐PTS converted to any fungitoxic substance, not already present in the aqueous solutions administered.