Three new pairs of biologically active regioisomeric toxins (AAL toxins TC, TD, and TE) were isolated from liquid cultures of Alternaria alternata f. sp. lycopersici, purified using standard chromatographic procedures, and their structures elucidated following interpretation of spectra obtained from NMR and mass spectrometry experiments. Each of the toxin congeners is structurally similar to the AAL toxin TA, which was characterized earlier from culture filtrates of this fungus. AAL toxin TC resembles TA but differs in its lack of hydroxyl groups at C4 and C5. AAL toxin TE is the N-acetylated form of TC. Spectroscopic data are reported to confirm that TB is similar to TA but lacks a hydroxyl group at C5, as suggested earlier. AAL toxin TD is the N-acetylated form of TB. All five pairs of regioisomers arise as products of fungal biosynthesis and do not appear to be generated during isolation. All regioisomeric pairs induce the genotype-specific necrosis characteristic of AAL toxin TA in tomato bioassays but differ markedly in relative toxicity.