The guaA gene encoding GMP synthetase was cloned from a potential probiotic strain of Lactobacillus rhamnosus. DNA sequence and Northern blot analysis indicated that (i) guaA did not belong to an guaAB operonic structure, conversely to enteric bacteria, (ii) L. rhamnosus guaA seemed to be highly expressed, and (iii) genetic regulation might differ from Bacillus subtilis. Moreover, differences in the genetic organization of guaA allowed discrimination of some closely related L. rhamnosus strains, with a rapid screening by PCR.