SUMMARY The predominant bacteria in canine urinary tract infections are derived from the bowel and the skin and usually ascend the urinary tract. Although they normally colonise the urethra and frequently enter the bladder, natural defence mechanisms exist to prevent their multiplication. Of these bladder wash-out is the primary defence, and consequently factors which interfere with urine voiding favour the establishment of infection. Pathogenic bacteria may produce cystitis and if infected urine is refluxed from the bladder to kidney acute or chronic pyelonephritis may result.