AbstractStructure‐activity relationships of the phytotoxins stemphyloxin I and II fromStemphylium botryosumf. sp.lycopersiciwere investigated by quantitative comparison of their biological activity with chemically related phytotoxins fromPhoma betae, betaenones A, B and C. Phytotoxicity was estimated by inhibition of incorporation of14C‐leucine into proteins of exponentially growing tomato cells. The values of 50 % inhibition for stemphyloxin I and II and for betaenones A, B and C were 0.075, 16, 55, 350 and 1 μM respectively. The β‐ketoaldehyde moiety appeared to be essential in, conferring biological activity and its toxicity was influenced by its spatial orientation. The presence of an hydroxyl group in stemphyloxin I and II enhanced activity in comparison with the respective compounds betaenones C and A which lack thi