Teleocidins produced by Actinomycetes are profoundly interesting indole alkaloids because of their tumor-promoting activity and unique structures with a nine-membered lactam ring and monoterpenoid moiety. A new metabolite named blastmycetin D (6 mg) was isolated from the mycelia (80 L) of Streptoverticillium blastmyceticum NA34-17, a strain known to produce Epstein-Barr virus early antigen-inducing indole alkaloids. The structure of blastmycetin D was elucidated using high-resolution electron ionization mass spectrometry (HR-EI-MS), UV, IR, NMR, and circular dichroism (CD) spectroscopy, establishing its molecular formula as C29H45N3O3, confirming the presence of an indole chromophore, a lactam ring, and an ether linkage, and revealing an absolute configuration consistent with lyngbyatoxin A at C-9, C-12, and C-19. Chemical conversion of blastmycetin D to olivoretin A suggested it is a possible precursor of olivoretin A. Isotope labeling experiments with L-methionine-methyl-¹³C-methyl-d3 showed that the C-23 methyl group of teleocidin B-4 originated from L-methionine, supporting a biosynthetic pathway for teleocidins involving methylation and oxidation at C-22 of a lyngbyatoxin A-type compound followed by intramolecular cyclization. Elucidation of the absolute configuration at C-22 of blastmycetin D and further investigation on the biosynthesis of teleocidins are in progress.