Two new peptaibols, namely microbacterins A (1) and B (2), were isolated from the deep sea inhabited actinomycete Microbacterium sediminis spp. nov. YLB-01(T). The sequences of the amino acid residues were determined on the basis of intensive NMR and ESI-MS/MS spectroscopic analysis, in addition to the Marfey's method and CD and optical rotation data for the configurational assignment. Both 1 and 2 exhibited significant cytotoxic activities against a panel of human tumor cell lines.