A long chain aldehyde-forming enzyme (LCAE, alpha-oxidation enzyme) in the marine green alga, Ulva pertusa, was purified 53-fold by differential centrifugation and chromatography with a DEAE Cellulofine A-500 column. The optimum pH of the purified LCAE ranged from 8.5 to 9.0. Linoleic, alpha-linolenic and palmitic acids were good substrates for the LCAE. Imidazole and glucose oxidase inhibited the activity strongly.