Abstract The effects of supplementation with clinoptilolite, a mineral from sedimentary deposits, were evaluated in a controlled and randomized field study conducted on a sheep farm with a known history of coccidiosis. Two groups of 6 pregnant ewes each, with three replicates per group (3 experimental and 3 control groups) were included in the study. Sheep were kept in pens and given free access to a basal diet or a clinoptilolite diet formulated by supplementation of the basal diet with clinoptilolite at a level of 1.25% and feed intake was determined on a daily basis. Clinoptilolite was fed to the experimental group for a mean of 72 days (30 ± 1.7 days before lambing through 42 days after lambing) and examination of fecal samples was performed every seven days in ewes as well as on days 12, 14, 21, 28, 35, and 42 in their lambs. The assessment of supplementation efficacy was based mainly on total oocyst excretion of Eimeria oocysts in ewes and their offspring, and on morphology of Eimeria oocysts. On day 42, efficacy reached a significant ( P ≤ 0.05) reduction of 97% in oocyst output in supplemented ewes in contrast with control animals. Lambs born from supplemented ewes showed a reduction of 98% in oocyst output in a mean of 28 ± 2.14 days after birth. Oocysts kept under laboratory sporulation conditions in 2% aqueous potassium dichromate as standard for optimal sporulation. The proportion of sporulated oocysts was determined microscopically every 12 h. Exposure to clinoptilolite in vitro or in vivo did not affect significantly the sporulation of oocysts shed by naturally infected sheep. However, 72% and 38% of sporulated oocysts isolated from supplemented and non-supplemented sheep, respectively, were deemed microscopically as damaged and seen to collapse when incubated in the presence of clinoptilolite. Oocysts from control ewes remained morphologically undamaged. These results suggest that clinoptilolite could reduce fecal shedding of infective oocytes by ewes which would result in less environmental contamination and lower the incidence of infection in suckling lambs under field conditions.