Acid resistance is considered an important virulence factor of the human pathogen Helicobacter pylori. The enzyme urease plays an important role in this acid resistance, but there are indications that other systems are present. We set out to establish the relevance of these urease-independent acid-resistance systems for growth at low pH. Four mutants out of a total of 1000 UV-mutants were urease positive, grew identical to wild-type on pH 7 plates, but did not grow on pH 5 plates. Whereas transformation of a mutant with its own chromosomal DNA did not restore growth at pH 5, transformation with wild-type DNA or DNA of one of the other mutants did restore the growth. From these complementation studies, we conclude that in H. pylori a urease-independent acid-resistance system, probably depending on the expression of more than one gene, is involved in the growth at low pH.