The daily variation of urinary excretion of Schistosoma haematobium and Schistosoma mansoni ova was assessed in a group of 24 Sudanese schoolboys using the filtration Trypan blue staining technique. An intra-class correlation coefficient of r = 0.88 indicated a surprisingly low daily variation of excretion of S. haematobium ova. All patients with a median egg output of at least 100 ova per 10 ml also voided S. mansoni ova in the urine. The daily variation of S. mansoni ova was slightly greater than for S. haematobium. For an excretion rate of at least five ova per 10 ml, the lower range of mean (P = 0.95) is one, if urine filtration is done on a single day. A circadian rhythm of S. haematobium egg excretion, with a peak around noon, was confirmed. Physical exercise combined with fluid intake prior to micturition significantly increased the egg output at all time points examined.