In our continuing work to search for phytotoxic substances produced by Pyricularia oryzae Cavara, we isolated five new compounds [3,4-dihydro-3,4,6,8-tetrahydroxy-1(2H)-naphthalenone (III), 3,4-dihydro-4,6,8-trihydroxy-1(2H)-naphthalenone (IV), and three pyriculol-related compounds V, VIa, VIb] related to 3,4-dihydro-3,4,8-trihydroxy-1(2H)-naphthalenone (I) and pyriculol (II), as well as known compounds 2-carboxy-3,5-dihydroxybenzylmethyl ketone (VII), 3,4-dihydro-6,8-dihydroxy-3-methyl isocoumarin (VIII), and prolylleucyl anhydride (IX) via repeated silica-gel column chromatographies of the ethyl acetate extract from the fungus culture broth. Structural elucidations of the new compounds were conducted using spectral data (UV, IR, NMR), spin-spin decoupling technique, chemical reactions (e.g., sodium borohydride reduction of pyriculol to confirm V's structure), and microbial transformation (conversion of pyriculol to V, VIa, VIb by P. oryzae, providing biosynthetic and stereochemical evidence for the C10-C11 α-glycol moiety). The α-glycol moiety of pyriculol and related compounds was determined to have a threo configuration via Corey's procedure (cyclic thiocarbonate formation and cis elimination to an olefin with cis configuration). Biological activity assays showed varying effects on rice seedlings: III had no significant activity; IV and IX slightly promoted second leaf growth at 500 ppm; VII and VIII inhibited root growth at 500 ppm but stimulated root elongation at 60 ppm; V, VIa, and VIb promoted root elongation at 250 ppm.