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A multicentric, placebo-controlled, randomised, blinded and blocked field study was conducted to evaluate the efficacy and safety of toltrazuril (Baycox, Bayer AG, Leverkusen, Germany) in the treatment of coccidiosis in first-year grazing calves naturally infected with Eimeria spp. Three-hundred and thirty-one calves were enrolled in the study and allocated to one of two treatments at a ratio of 1:1. One hundred and sixty-seven animals were treated once orally with 15 mg/kg toltrazuril, and 164 animals served as placebo-treated controls. Two treatment regimes were compared, a metaphylactic (treatment on the day, or 1 day after, turn out) and a therapeutic treatment (4 or 7 days after turn out). During an observation period of 14 days after treatment the animals were clinically examined for diarrhoea and faecal samples were regularly assessed for Eimeria oocysts. Other possible causes of diarrhoea were excluded on the basis of microbiological and virological examination. Animals were predominantly infected with Eimeria alabamensis. Number of days with diarrhoea in animals treated with toltrazuril was significantly lower compared to the placebo-treated group (therapeutic treatment: P=0.0024; metaphylactic treatment: P<0.0001). Furthermore, the number of animals with diarrhoea during the observation period for a minimum of at least 3 days, the number of animals positive for Eimeria oocysts, and the number of animals with both diarrhoea for a period of at least 3 days and positive for Eimeria oocysts, were significantly lower (P<0.01), in the toltrazuril- compared to the placebo-treated animals. Body weight in the toltrazuril-treated animals significantly exceeded that of the placebo-treated animals at the end of the observation period. Mean difference in body weight was higher in the metaphylactic (+7.3 kg) compared to the therapeutic treatment group (+3.4 kg). No adverse reactions were observed. The results indicate that toltrazuril is highly efficacious and safe in the metaphylactic and therapeutic treatment of coccidiosis caused by E. alabamensis in first-year grazing calves.