L-Arabinose-negative mutants of Escherichia coli B/r, ara-53 and ara-139, are deficient in the enzyme L-ribulose 5-phosphate 4-epimerase; ara-53, further analyzed, accumulates large quantities of L-ribulose 5-phosphate when incubated with L-arabinose. The mutant sites are closely linked to the left of the previously ordered L-arabinose mutant sites, and probably represent the structural gene for L-ribulose 5-phosphate 4-epimerase (gene D) in the L-arabinose operon. The inducible levels of L-arabinose isomerase and L-ribulose 5-phosphate 4-epimerase vary correspondingly as a result of mutation in the structural gene for L-ribulokinase (gene B), further substantiating the dual structural and regulatory function of this gene locus. Ara-53 and ara-139 are strongly inhibited by L-arabinose and give rise to L-arabinose-resistant mutants. The one resistant mutant analyzed still lacks the 4-epimerase but is deficient in L-ribulokinase and has increased L-arabinose isomerase activity, a characteristic of a type of mutation in the B gene. It is proposed that accumulation of L-ribulose 5-phosphate is responsible for the inhibition, and that mutation to resistance will involve mutation in the A, B, C, permease, or repressor genes, thus providing a direct method for isolating these types of L-arabinose-negative mutants. Glucose prevents and cures the L-arabinose inhibition.