Klebsiella aerogenes NCIB 418 assimilates glycerol via alternative pathways: one involves a glycerol kinase with a high affinity for glycerol (apparent Km = 1-2x10(-6)M), and the second a glycerol dehydrogenase with a much lower affinity for its substrate (apparent Km=2-4x10(-2)M). In variously-limited chemostat cultures, one or the other pathway predominated. Thus, aerobic carbon-limited organisms contained only the glycerol kinase pathway whereas aerobic sulphate-limited or ammonia-limited organisms (grown on glycerol) used only the glycerol dehydrogenase pathway. Anaerobic cultures invariably contained glycerol dehydrogenase, and glycerol kinase was absent. Washed suspensions of aerobically-grown organisms oxidized glycerol with kinetics similar to that of the particular enzyme (the primary enzyme of the assimilatory pathway) which they possessed, thus indicating a close association between these two enzymes and the uptake process. But a supply of exogenous glycerol was not a prerequisite for the synthesis of either glycerol kinase or glycerol dehydrogenase, and nor was molecular oxygen the key factor in effecting modulation between the alternative pathways of glycerol metabolism, as had been previously suggested. The physiological significance of dual pathways of glycerol assimilation is discussed.