Zamamistatin was isolated from the Okinawan sponge Pseudoceratina purpurea. It was determined to be a novel bromotyrosine derivative by careful analysis of 2D NMR spectra and comparison of its 1 H NMR spectrum with those of structurally related compounds. The absolute stereostructure of zamamistatin was determined by the modified Mosher's method. Zamamistatin exhibited significant antibacterial activity against Rhodospirillum salexigens, which has adhering properties.