A series of mutations are described which map in the araB gene of the l-arabinose operon and exert a polar effect on gene araA, the structural gene for the l-arabinose isomerase. Ten of the 20 araB point mutants examined exhibited absolute polarity and may represent insertions of genetic material into the araB gene. The remaining 10 point mutants exhibit strong polarity (less than 10% of the normal wild-type inducible level of isomerase) and may represent a class of externally suppressible polar mutations other than amber or ochre. Seven of the 12 araB deletion mutants examined, or 58%, exhibit polarity, suggesting that a shift in the reading frame has been generated in the polycistronic message for the l-arabinose operon. The remaining, presumably in-phase, deletion mutants exhibit hyperinducible levels of isomerase, an effect that is eliminated when an araB(+) gene is introduced in the trans position. The hyperinducibility effect is discussed in terms of a model for self-catabolite repression, originally proposed by Katz and Englesberg.