Urease-producing bacteria have been shown to affect the formation of infection stones by splitting urea into ammonia, bicarbonate and carbonate. An increase in alkaline pH results in urinary supersaturation of the ions. The increase in ammonia also causes injury to the urothelial glycosaminoglycan layer. Non-urease-producing bacteria have been speculated to form urinary stones. Midstream voided bladder urine and fractured stone nidus samples from 72 patients undergoing surgery for urolithiasis were cultured on specific media for genital mycoplasmata and on conventional media. Urine samples were obtained from a control group of 40 healthy subjects. Genital mycoplasmata and other bacteria were evaluated with regard to the composition of urinary stones. Compared with other origins of stones, the relation between isolation of Ureaplasma urealyticum and infection stone disease was statistically proven. Isolation of genital mycoplasmata was significantly higher in women than in men in the study group. The urinary stones comprised 84.7% calcium stones, 8.3% uric acid stones and 6.9% infection (magnesium ammonium phosphate) stones. Coagulase-negative Staphylococci, Escherichia coli, Corynebacterium spp., Enterobacterium spp. and U. urealyticum were cultured from stone samples. The results suggests that non-urease-producing bacteria, as well as urease-producing bacteria, may influence the formation of urinary stones.