A field study was conducted to evaluate the effect of a single oral treatment with 20 mg/kg body weight (BW) of toltrazuril (Baycox 5% suspension)--TOL--in comparison to a single oral treatment with 1 mg/kg BW of diclazuril (Vecoxan suspension orale, 2.5 mg/ml)--DIC--and an untreated control group (CTRL) on naturally acquired Eimeria infections in lambs. On a French sheep farm with a known history of coccidiosis, 75 housed lambs aged 10-14 days were randomised and allocated to one of three groups. During an observation period of 60 days after treatment, clinical (faecal consistency, BW) and parasitological parameters (oocyst excretion) were evaluated. Excretion in the negative control group started 3 days after treatment and peaked on the 31st day with a prevalence of 80%. Animals were predominantly infected with Eimeria ovinoidalis. Treatment with toltrazuril, but not with diclazuril, resulted in significantly reduced numbers of excreting animals. The number of excretion days and the average oocyst excretion decreased significantly in both the TOL and the DIC groups compared to the CTRL, with the TOL group showing significantly fewer excretion days and excretion intensities than the DIC group. Changes in the faecal consistency were moderate throughout the study and not significantly different between the groups. Daily weight gains were higher in the TOL group compared to the DIC and CTRL groups which did not differ. This study demonstrates the good efficacy of toltrazuril administered orally to lambs in the prepatent period in subclinical natural Eimeria infections in housed lambs.