Bacillamide (1) was isolated as a new algicide against the harmful dinoflagellate, Cochlodinium polykrikoides, from the marine bacterium, Bacillus sp. SY-1, and its structure was elucidated by extensive two-dimensional NMR techniques including 1 H–15N HMBC analysis and mass analysis. Bacillamide showed algicidal activity against C. polykrikoides with LC50 of 3.2 g/ml.