A cytotoxic tetramic acid lactam named cylindramide (1) has been isolated from the marine sponge Halichondria cylindrata. The structure was determined by interpretation of 2D NMR spectra as a macrocyclic lactam including an acyltetramic acid and a trisubstituted bicyclo[3.3.0]octene. This lactam was cytotoxic against B16 melanoma cells with an IC50 Of 0.8 mug/mL.