The absolute stereostructure of a potent cytotoxic dimeric macrolide misakinolide A (5)(=bistheonellide A), which was isolated from an Okinawan marine sponge of Theonella sp., was determined by chemical correlation with swinholide A (1). The stereochemistry of misakinolide A was shown to be identical with tha of another cytotoxic dimeric macrolide, swinholide A (1), which was isolated from the Okinawan marine sponge Theonella swinhoei and the stereostructure of which was recently determined.