The immunogenicity and cross-protection of Eimeria maxima or E. tenella in chickens against their homologous and heterologous strains were evaluated in two experiments. The immunizing strain of E. tenella protected against itself and partially against heterologous strains. The North Carolina (NC) strains of E. tenella were more virulent than the Delmarva (DMV) strains. Growth of the unimmunized groups was depressed 66% and 32% for the NC and DMV strains, respectively. Growth of the immunized-challenged groups (DMV and NC) was depressed by 13%. The DMV E. maxima strains were more virulent than the NC strains. Growth of the unimmunized challenged groups was depressed by 47% (DMV) and 13% (NC). Results demonstrated that there are antigenic variations among strains of two species of chicken coccidia.