The relationship between environment and mutation is complex []. Claims of Lamarkian mutation [] have proved unfounded [[3–5]]; it is apparent, however, that the external environment can influence the generation of heritable variation, through either direct effects on DNA sequence [] or DNA maintenance and copying mechanisms [[7–10]], or as a consequence of evolutionary processes [[11–16]]. The spectrum of mutational events subject to environmental influence is unknown [] and precisely how environmental signals modulate mutation is unclear. Evidence from bacteria suggests that a transient recombination-dependent hypermutational state can be induced by starvation []. It is also apparent that chnages in the mutability of specific loci can be influenced by alterations in DNA topology [[10,17]]. Here we describe a remarkable instance of adaptive evolution in Salmonella which is caused by a mutation that occurs in intermediate-strength osmotic environments. We show that the mutation is not ‘directed’ and describe its genetic basis. We also present compelling evidence in support of the hypothesis that the mutational event is constrained by signals transmitted from the external environment via changes in the activity of DNA gyrase.