In general, isocoumarins have been reported to exhibit antibacterial, antitumor and gastroprotective activity and some have been isolated from culture broths of the genus Bacillus. In our screening program for new antitumor compounds produced by marine bacteria, we have isolated a new isocoumarin named PM-94128 from the culture broth of Bacillus sp. PhM-PHD-090 isolated from a marine sediment. PM-94128 is a basic substituted isocoumarin antibiotic with cytotoxic activity against several tumor cell lines.