Oocysts of C. muris and the events of excystation using 0.5% sodium hypochlorite as excystation medium were described with light microscope. The response of the immunocompetent BALB/c mice against infection was studied using sera of orally infected mice at different periods postinoculation by indirect immunofluorescence antibody test using 1:150 FITC conjugated rabbit serum antimouse polyclonal IgG. From the patterns of IFAT, it was suggested that the dominant antigen in C. muris was restricted to the apical complex of the sporozoites. Such antigen may play a role in the invasion of the host cell. Future analysis of such receptor molecules might constitute prime candidates as immunogens for a vaccine, the efficiency of which might cause inhibition of parasite invasion.