The conversion of beta-carotene to retinal by a recombinant beta-carotene 15,15'-dioxygenase (Blh protein) from an unculturable marine bacterium was optimized in aqueous solution. Toluene was optimal solvent for the dissolution of beta-carotene and the optimal solution for the conversion reaction contained 2.4% (w/v) Tween 20, 0.15 U enzyme/ml, and 350 mg beta-carotene/l. Under these conditions, the enzyme produced 181 mg retinal/l after 20 h. This is the highest reported value for the retinal concentration from beta-carotene.