Algal transformation of racemic norbornanone [(+/-)-2-norbornanone] by Chlorella pyrenoidosa Chick was investigated. (+/-)-2-Norbornanone was preferentially reduced to (-)-endo-norborneol which was obtained as a product containing 8% of exo-isomer, with a constant rate of endo- vs. exo-alcohol (82:18) in the media detected up to 48 h. The yield and accumulation ratio of the alcohols were not affected over the 5.5 to 10.5 pH range tested. Addition of allyl alcohol inhibited the yields of endo-norborneol more efficiently than that of exo-isomer. Differences in the reductive activity of (+/-)-2-norbornanone were observed among three strains of Chlolella in which C. pyrenoidosa Chick possessed the highest activity.