Cryptosporidium parvum infection was detected in 137 of 183 dairy calves up to 12 weeks of age (74.8%) on a convenience sample of 30 farms with a history of calf diarrhoea. The prevalence was determined by using a formol-ether concentration and acid fast staining method (Ziehl Neelsen). AU farms studied had infected calves at the time of sampling and within farm prevalence ranged from 25 to 100%. The higher percentages of oocysts excreted were recorded in calves with 7 to 14 days (89%) and 15 to 21 days old (90%). Eighty-four of the calves were diarrhoeic (46%) and 74 of these calves were shedding C. parvum oocysts (88%). Cryptosporidiosis appears to be an important cause of dairy calf scours in this region. A second study was performed on 79 newborn calves from the later farms to assess the efficacy of halofuginone lactate (Halocur®) in the prevention of cryptosporidiosis. Halocur® was administered at 120μg/Kg/day for 7 days to 39 calves and the other 40 calves did not receive any antiprotozoal treatment. Over a period of 3 weeks the animals were examined four times for shedding of C. parvum oocysts and diarrhoea. The intensity of infection was determined by a semi-quantitative score of oocyst numbers in faeces. Oocyst excretion peaked on Day 7 and Day 14 in the control group (100% and 94%) being at the same time lower in the treated halofuginone-group (30.5 % and 55.6 %). During that period intensity of shedding was significantly lower (P<0.0001) in the treated group, thus contributing to decrease environmental contamination and zoonotic risks. Halocur® reduced significantly the number of calves presenting diarrhoea on Day 14 (P<0.001). Despite the low intensity of shedding on Day 7, the high prevalence of diarrhoea of the treated group (58%) may suggest the presence of other enteropathogens as the cause of scouring, such as Escherichia coli K99, rotavirus and coronavirus.