Agropyrenol is a phytotoxic substituted salycilic aldehyde produced in liquid culture by Ascochyta agropyrina var. nana, a potential mycoherbicide proposed for the control of perennial weed Elytrigia repens. In this study, six derivatives obtained by chemical modifications of the toxin were assayed for phytotoxic, antimicrobial and zootoxic activities, and the structure-activity relationships were examined. Each compound was tested on non-host weedy and agrarian plants, fungi, Gram+ and Gram- bacteria, and on brine shrimps larvae. The results provide insights into the structure-activity relationships of agropyrenol. Both the double bond and the diol system of the 3,4-dihydroxypentenyl side chain, as well as the aldehyde group at C-6 and C-1 of the phenolic ring of agropyrenol proved to be important for the phytotoxicity. The lesser polar 3',4'-O,O'-isopropyledene of agropyrenol showed also a significant zootoxic and a slight antimicrobial activity. This finding could be useful in devising new natural herbicides for practical application in agriculture.