A strain of Ascochyta agropyrina var. nana was isolated from Elytrigia repens (quack grass), a noxious perennial weed widespread through the cold regions of the northen and southern hemispheres. Papyracillic acid was isolated for the first time from the fungal solid culture and identified using spectroscopic methods, including X-ray diffractometric and CD analysis for the assignment of the relative and absolute stereochemistries. Some key derivatives were prepared and used in a structure-activity relationship study. Tested by leaf disk-puncture assay, papyracillic acid at the concentration of 1 mg/mL was shown to be phytotoxic both for the host plant and a number of nonhost plants of the fungus. Papyracillic acid was active against bacteria (Xanthomonas campestris and Bacillus subtilis) and the fungus Candida tropicalis at 6 microg/disk. Derivatives of papyracillic acid were significantly less active than original toxin. However, the monoacetyl derivative of the toxin did not possess antimicrobial activity but remained highly phytotoxic to quack grass. Hence, papyracillic acid and its analogues have potential as nonselective herbicides of natural origin. Some structure-activity relationship observations for papyracillic acid and its derivatives were also made.