All members of this Phylum are obligatory parasites. The phylum contains the coccidial parasites and important blood parasites of cats. Two of the parasites Toxoplasma gondii and Cryptosporidium parvum are of public health importance because they are zoonotic agents. The phylum obtains its name from the assemblage of organelles that are present in the anterior end of invasive stages and collectively form the apical complex (Current et al., 1990). The apical complex is involved in the entrance of parasites in to host cells. Cats are definitive hosts for several apicomplexan parasites including the genera Isospora, Toxoplasma, Hammondia, Besnoitia, Sarcocystis, Cryptosporidium, Babesia, and Theileria (syn. Cytauxzoan). Isospora and Cryptosporidium are coccidial parasites that can be associated with diarrhea. Toxoplasma, Hammondia, Besnoitia, and Sarcocystis are coccidial parasites that have a two-host life cycle and use cats as definitive hosts. Intestinal infections in cats with these species are usually asymptomatic. Babesia and Theileria species are tick-transmitted blood parasites that can cause anemia and death.