The marine bacterium, Bacillus sp. SY-1, produced algicidal compounds that are notably active against the bloom-forming alga Cochlodinium polykrikoides. We isolated three algicidal compounds and identified these as mycosubtilins with molecular weights of 1056, 1070, and 1084 (designated MS 1056, 1070, and 1084, respectively), based on amino acid analyses and ¹H, ¹³C, and two-dimensional nuclear magnetic resonance spectroscopy, including ¹H-¹⁵N heteronuclear multiple bond correlation analysis. MS 1056 contains a β-amino acid residue with an alkyl side chain of C₁₅, which has not previously been seen in known mycosubtilin families. MS 1056, 1070, and 1084 showed algicidal activities against C. polykrikoides with 6-h LC₅₀ values of 2.3 ± 0.4, 0.8 ± 0.2, and 0.6 ± 0.1 μg/ml, respectively. These compounds also showed significant algicidal activities against other harmful algal bloom species. In contrast, MS 1084 showed no significant growth inhibitory effects against other organisms, including bacteria and microalgae, although does inhibit the growth of some fungi and yeasts. These observations imply that the algicidal bacterium Bacillus sp. SY-1 and its algicidal compounds could play an important role in regulating the onset and development of harmful algal blooms in natural environments.