We have described a rapid spontaneous conversion in the stationary phase of Escherichia coli strain DOO (crp ^-) cells as a whole population to crp⁺ state (Sugino and Morita, 1994). In this paper we have tried to elucidate the molecular basis of this unidirectional conversion by cloning and sequencing of the crp gene in their crp⁺ and crp^- states. We have found that in the original crp^- strain, an IS2 element has been inserted between its original promoter and the coding region of the crp gene in the so-called orientation II (Ahmed et al., 1981), accompanied by an 11 bp deletion. Unexpectedly, the crp ⁺ "revertants" derived from the crp^- mutant had no difference in sequence from the crp^-, either in the coding or the regulatory region. This suggests that a change at another locus, such that this change somehow activates the expression of the crp gene to the level of a normal crp ⁺, is responsible for the apparent reversion from crp ^- to crp⁺.