Large numbers of oocysts of Eimeria alabamensis have been found in the faeces of calves suffering from diarrhoea shortly after being turned out to pasture. To investigate the source and clinical significance of this coccidial infection, the numbers of oocysts excreted, the consistency of the faeces and the growth rates of four groups of 12 calves were compared. Group I calves were kept indoors and their diet was unchanged, Group II calves were turned out onto a previously ungrazed pasture, Group III calves were turned out onto a permanent pasture and Group IV calves were kept indoors and fed cut grass from a previously ungrazed field. Eight days after the animals were turned out there was an almost 1000-fold increase in the numbers of oocysts in the faeces of Group III calves, the dominant species being E. alabamensis, but there were only minor fluctuations in the numbers of oocysts excreted by the other groups. It was therefore concluded that the source of the infection was oocysts that had overwintered on the permanent pasture. Most of the calves in Group III developed watery diarrhoea 5 days after turnout, but there was only a slight softening of the faeces of the calves in Groups II and IV at about the same time. The faeces of the calves in Group I was of firm consistency throughout the trial. The calves in Group III lost 18 kg during the 24 day period following turnout, whereas the calves in the other groups gained between 6 and 18 kg.