Anaerobic growth of Escherichia coli on L-fucose or L-rhamnose as the sole source of carbon and energy depends on the regeneration of NAD from NADH by disposing the intermediate L-lactaldehyde as L-1,2-propanediol. The two parallel pathways, with their own permeases and enzymes encoded by two widely separated gene clusters, appear to share a single enzyme that catalyzes the formation of L-1,2-propanediol. Although this oxidoreductase is encoded by a gene at the fuc locus, the enzyme is inducible by both L-fucose and L-rhamnose. The inducibility by L-rhamnose is controlled by a gene at the rha locus with no other known functions, since the aerobic growth rate on L-rhamnose remains normal. L-1,2-Propanediol oxidoreductase activity is inducible only anaerobically, and the effect of the two methylpentoses operates at different levels: L-fucose exerts its influence post-transcriptionally; L-rhamnose exerts its influence transcriptionally.