Goussia carpelli causes enteritic coccidiosis in juvenile carp. In nature it affects carp fry (1 to 2 mo old) and fish (3 to 4 mo old) which are subjected to environmental stress. Carp treated with corticosteroids or subjected to temperature stress in the laboratory produced higher numbers of oocysts during the primary infection. Resistance to reinfection via the fecal contamination route, however, was not reduced by the application of hydrocortisone, dexamethasone, or X-ray irradiation given both prior to and concurrently with reinfection. The administration of hydrocortisole or irradiation did not induce a relapse of a previous infection. Carp which had been immunosuppressed by hydrocortisole injection during a primary infection were also refractory to a secondary infection via fecal contamination. The results of these experiments suggest that the mechanisms which are responsible for the resistance of carp to secondary infections with Goussia carpelli were not affected by hydrocortisole, dexamethasone or X-ray treatment.