An outbreak of bilharzia (Schistosoma mattheei) infestation involving about 100 oxen on the farm Otthilie near Tolwe in northern Transvaal is described. Infestation of the cattle appears to have occurred per os from a single drinking trough. The cattle showed clinical signs and pathological lesions of both the reinfestation and the acute syndromes. The worm burdens were the highest recorded hitherto in naturally infested cattle. Severe macroscopic clay pipe stem periportal fibrosis, granulomata of the ureters and severe grey pigmentation of the lungs, are described for the first time in naturally infested cattle. There was more marked liver pigmentation than had been encountered previously in this host. A striking feature in every case was conspicuous congestion and enlargement of the ileo-cecal valve, which was dark red in colour. Treatment of affected animals with injectable trichlorphon controlled the outbreak without causing mass embolism and liver infarcts. At least 11 treatments at a dosage level of 10-12 mg/kg per treatment at 3-5 day intervals are necessary. Faecal examinations for eggs or miracidia cannot be used to assess the worm burden or the clinical state of the animal. Moreover, it is suggested that this disease may be confused with other conditions and this probably accounts for the rarity of reports of outbreaks in ruminants.