The absolute stereochemical configuration of the phytanyl (3,7,11,15-tetramethylhexadecyl) groups in the lipids of Halobacterium cutirubrum was determined as follows. Conversion of the phytanyl groups to phytanic and pristanic acids yielded (+)-3,7,ll,15-tetramethylhexadecanoic acid and (−)-2,6,10,14-tetra-methylpentadecanoic acid, respectively, showing that C-3 in the phytanyl group has the d or R configuration. Reduction of the pristanic acid to pristane (2,6,10,14-tetramethylpentadecane) gave the meso-6R,10S isomer, identical with that obtained from phytol, showing that C-7 and C-ll in the phytanyl groups both have either the R,R or the 5,5 configuration. Barbier-Wieland degradation of the pristanic acid yielded a C18 ketone identical with (6R,10R)-14-trimethylpentadecanone-2, showing that C-7 and C-11 in the phytanyl groups have the R configuration. These findings thus establish the absolute configuration of the bacterial phytanyl groups as (3R,7R,11R)-15-tetramethylhexadecyl. The bearing of these findings on the configuration of phytanyl derivatives from various sources is discussed.