Studies on the activation and excystation of I. felis and I. rivolta sporozoites revealed that, although some differences were apparent between the two, both were capable of activation under a wide range of conditions. Activation of both species took place in trypsin and bile between 21 and 43 C (the range tested) occurring rapidly at 39 C. Although the presence of bile appeared to be essential for this process that of trypsin did not. Neither the concentration of bile (above 5%), the type of bile, nor the hydrogen ion concentration had a marked effect on the level of activation. Pretreatment of oocysts in aqueous cysteine hydrochloride under CO2 or with sodium hypochlorite before exposure to trypsin and bile was not an essential prerequisite for the activation of I. felis and I. rivolta. However, higher levels of activation were attained when pretreatment was used. But for I. rivolta, the level of activation appeared to be less dependent on pretreatment for oocysts stored in sulfuric acid than for those stored in potassium dichromate. The process of activation and excystation of both species was essentially similar to that described for other species of disporocystic coccidia with heteroxenous life cycles. Sporozoites escaped following the collapse of the sporocyst wall and completed excystation through indentations and fractures at one or both ends of the oocyst.