The very rare occurrence of ascariasis of the kidney is reported in a 6-year-old boy from Cape Town, South Africa. He was acutely ill with pyonephrosis and passed adult Ascaris lumbricoides per urethra before having a nephrectomy. There was no evidence of a fistula between the intestines and the urinary system, and as the development of larvae into mature helminths within the renal system is not considered to be possible it is likely that mature worms gained entrance through the urethra. It appears that the worms obstructed the right ureter and caused hydronephrosis, pyonephrosis, and deposition of ova in renal tissue.