Ninety, seven- to 10-day-old calves were allocated to three groups of 30 and treated daily for seven days with either 100 μg/kg halofuginone hydrobromide or 2·5 mg/kg decoquinate orally or left untreated as controls. The levels of diarrhoea and dehydration were monitored daily for 28 days from the first day of treatment (day 0) and samples of faeces were collected on days 0, 7, 14, 21 and 28, to quantify the excretion of Cryptosporidium parvum oocysts. The calves were weighed on days 3 and 28. The treatments had no effect on the levels of diarrhoea or dehydration, the proportions of diarrhoeic calves or the proportions of calves shedding oocysts. However, unlike decoquinate, halofuginone significantly reduced the excretion of oocysts on day 7 (P>0·0001), and decoquinate increased the average daily weight gain of the calves (P=0·049).