Incorporation of C-13-labeled glycerol or pyruvate into the ubiquinone Q8 of Escherichia coli mutants lacking enzymes of the triose phosphate metabolism and of (U-C-13(6))glucose into the triterpenoids of the hopane series of Zymomonas mobilis showed that glyceraldehyde 3-phosphate (or eventually glyceraldehyde) and a C-2 unit derived from pyruvate decarboxylation were the only precursors of the C-5 skeleton of isoprenic units in a novel non-mevalonate pathway for isoprenoid biosynthesis in these bacteria.