The crystalline biofilms formed by Proteus mirabilis can seriously complicate the care of patients undergoing long-term bladder catheterization. The generation of alkaline urine by the bacterial urease causes calcium and magnesium phosphates to precipitate from urine and accumulate in the catheter biofilm, blocking the flow of urine from the bladder. The pH at which these salts crystallize from a urine sample, the nucleation pH (pHn), can be elevated by diluting the urine and by increasing its citrate content. The aim of this study was to examine whether manipulation of pHn in these ways modulated the rate at which crystalline biofilm developed. Experiments in laboratory models of the catheterized bladder infected with P. mirabilis showed that when the bladder was supplied with a concentrated urine (pHn 6·7) at a low fluid output (720 ml per 24 h), catheters blocked at 19–31 h. Diluting this urine 1 : 4 increased the pHn to 7·5 and models supplied with this urine at 2880 ml per 24 h took 110–137 h to block. When models were supplied with urine containing citrate at 1·5 mg ml−1 or above (pHn 8·3–9·1), the catheters drained freely for the full 7 day experimental period. Scanning electron microscopy revealed that the catheter biofilms that developed in urine with high pHn values were devoid of crystalline formations. These observations should encourage a clinical trial to examine the effect of increasing a patient's fluid intake with citrate-containing drinks on the encrustation and blockage of catheters.