The hopanoid products synthesized by two mutant strains of Methylobacterium together suggest a biosynthetic pathway for the formation of the hopanoid side chain. Mutants deficient in the gene hpnH lack side chains entirely, while those deficient in hpnG accumulate adenosylhopane. These results are in accordance with adenosylhopane as a precursor to extended hopanoids and suggest that adenine is subsequently cleaved, possibly forming phosphoribohopane. We propose that the great diversity of microbial bacteriohopanepolyols and composite hopanoids reflects processes occurring downstream of this intermediate.