The green marine alga Ulva lactuca is known to contain high concentrations of bromophenols. However, the biosynthetic pathways of their formation is not known. This study was aimed at identifying possible precursors of bromophenols. The bromophenol content and brornoperoxidase activity were measured in the alga collected every month from January to August 1997. Bromoperoxidases were extracted and incubated with various possible precursors of bromophenols and brominated reaction products were identified by gas chromatography-mass spectrometry. The results show that U. lactuca contains a brornoperoxidase which can convert phenol, 4-hydroxybenzoic acid and 4-hydroxybenzyl alcohol to bromophenols. L-tyrosine and 4-hydroxybenzaldehyde could not be converted to bromophenols. Bromophenol content and brornoperoxidase activity exhibited a seasonal variation with high productivity in summer and low in winter.