Guided by inhibitory activity against IMP dehydrogenase (IMPDH), a new brominated diphenylmethane derivative, isorawsonol, has been isolated from the tropical green alga Avrainvillea rawsonii. Its structure was determined by detailed spectroscopic analysis including HMQC and HMBC. The known metabolites, avrainvilleol, avrainvilleol methyl ether, 3-bromo-4,5-dihydroxylbenzyl alcohol, and rawsonol, were also isolated from this alga and two of these showed a modest ability to inhibit IMPDH. The formation of isorawsonol is envisioned as being similar to that of rawsonol involving condensation of two molecules of avrainvilleol.