Alkali production from urea by bacterial ureases in the oral cavity is thought to have a major impact on oral health and on the physiology and ecology of oral bacteria. Using continuous chemostat culture, urease activity in Streptococcus salivarius 57.I was examined as a function of growth pH, carbohydrate availability and growth rate. A portion of the S. salivarius ureC gene was amplified by polymerase chain reactions (PCRs) using degenerate primers encoding highly conserved sequences from known ureases. The nucleotide sequence of the PCR product was determined, and was used to compare the level of urease gene expression under different growth conditions. The data indicated that urease was highly expressed at low pH, and expression was also modulated by glucose availability and growth rate. Differential expression was controlled, at least in part, at the transcriptional level.