Infection with Eimeria sp. was studied in 135 lambs born to 79 ewes in four early lambing housed flocks. In three of these flocks two different coccidiostats (monensin and decoquinate) were administered in the feed. Cohort lambs were examined clinically and a rectal faeces sample taken once a week. Samples with more than 300 oocyts per gram were speciated by morphology. Nine species were identified and Eimeria crandallis was the most prevalent. One of the flocks developed clinical coccidiosis before the introduction of coccidiostats. On the remaining farms no differences in the oocyst excretion rate of infected lambs were found between medicated and non-medicated lambs until Visit 6 nor in the proportion of lambs infected until Visit 8. It is suggested that cocciodiosis may be controlled without coccidiostats, by identification of the risk factors associated with disease. Morphological variation of species and collection of large enough faeces samples from young lambs constitute limiting components for further epidemiological studies of coccidiosis.