Isolated nucleic acid molecules encoding coccidian casein kinase I, CKI, enzymes from the species Eimeria tenella and Toxoplasma gondii are disclosed. The isolation of these coccidian CKI cDNA sequences results in the disclosure of purified forms of E. tenella and T. gondii CKI proteins, recombinant vectors and recombinant hosts which express coccidian CKI.