A terrestrial bacterium, Streptomyces sp. NZ-6, produced niizalactams A-C (1-3), unprecedented di- and tricyclic macrolactams, by coculturing with the mycolic acid-containing bacterium Tsukamurella pulmonis TP-B0596. Their complete structures, including absolute configurations, were elucidated on the basis of spectroscopic data and chemical derivatization. Their unique skeletons are proposed to be biosynthesized from a common 26-membered macrolactam intermediate by SN2 cyclization or an intramolecular Diels-Alder reaction.