In the course of our search for new biologically active metabolites, lachnellin A (1), a metabolite with high cytotoxic and antimicrobial activities, the structurally related lachnellins B, C and D (3, 4, 7), and naphthalene-1,3,8-triol (8), an inhibitor of malate synthase (EC 220.127.116.11), were isolated from submerged cultures of the ascomycete Lachnellula sp. A 32 -8 9 . The antimicrobial, cytotoxic and phytotoxic activities of lachnellin A depended on its reactivity and could be abolished by the addition of cysteine. The enzyme inhibiting activity of (8) was due to reactive intermediates during melanization and was no longer observed in the presence of serum albumin. In addition, rac-scytalone (9), (+)-trans-3,4-dihydro-3,4,8-trihy-droxy-1(2H)-naphthalenone (10). 2,5-dihydroxytoluene (11), and (R)-(-)-5-methylmellein (12) were obtained from the same source and biologically characterized.