SYNOPSIS.Oocysts ofEimeria boviswere found to undergo excystation when subjected at 39 C to a pretreatment consisting of exposure for 24 hr to CO2and air (50–50), and a treatment for 7 hr with a mixture of bile and trypsin. At pH's of 6.0 thru 10.0 with tris‐maleate buffer, excystation occurred over the entire range of pH tested, with the highest levels at pH 7.5‐8.5. No adverse or inhibitive effect on excystation or the viability of the sporozoites was observed. Disintegration of sporozoites occurred within the sporocysts of intact oocysts at each of the pH levels studied when boric acid‐borax, ammediol, and glycine‐sodium hydroxide buffers were used in the treatment medium. Phosphate buffer inhibited excystation when used in the excysting medium. Excystation occurred at levels above 90% in all dilutions of taurocholic, glycocholic, glycotaurocholic, and cholic acids included in the study (0.5‐10.0%) except for the 10% and 5% dilutions of cholic acid and the 10% dilution of glycotaurocholic acid. In the latter 3 dilutions, sporozoites within the sporocysts of intact oocysts disintegrated. Excystation levels above 90% were observed in the 50% and 10% dilutions of fresh bovine bile, and in the 5% dilution of lyophilized bovine bile. Lower levels of excystation occurred in greater dilutions of both kinds of bile. No excystation occurred when any of the bile acids, fresh bovine bile or lyophilized bile were used without trypsin, except for fresh bile that contained a heavy suspension of bacteria and fungi. In a medium containing trypsin and heat‐treated bile, heat‐treated bile acids, or no bile, 2.5–8% of the oocysts excysted. The findings indicate that satisfactory excystation can be obtained with a treatment medium containing tris‐maleate at pH 7.5–8.5, 0.25% trypsin, and 1% of