D-Arabinose is a major sugar of the mycobacterial cell wall and occurs in two important cell wall polymers, an arabinogalactan and a lipoarabinomannan. In spite of the widespread occurrence of this sugar in this group of organisms, very little is known about its pathway of biosynthesis. Previous studies from our laboratory and that of McNeil et al, in which mycobacterial cells were incubated with [1-(14)C]- or - [6-(14)C] glucose indicated that both labels were equally good precursors of radioactive arabinose, suggesting that this sugar arises via the degradation and reassembly of a hexose precursor, rather than directly by the hexose-monophosphate shunt.