Algal transformations of four alicyclic ketones were photoautotrophically studied with an axenic strain of Chlorella pyrenoidosa Chick. The algal reduction of 2-, 3- and 4-methylcyclohexanones, and 4-tert-butyl-cyclohexanone to form the corresponding cis- and trans-alcohols were confirmed by GC-MS analysis. The cis/trans ratios of the resulting cyclohexanols were different from those obtained by reduction with Aspergillus repens. The ratio of cis-isomer to trans-one of 3-methylcyclohexanol was especially large (20 : 1).