The kinetics of Ig-positive cell populations in carp tissues was followed during an infection with the gut dwelling coc-cidian Goussia carpelli Léger et Stankovich, 1921. In cell suspensions of the anterior and posterior sections of the intestine, the proportion of Ig-positive cells increased with the development of the coccidia and peaked during oocyst formation at day 15 post exposure. These results suggest a reaction of the local mucosal immune system. In cell suspensions of pronephros the proportion of Ig-positive cells increased as well, indicating that a systemic immune response was also induced against this intestinal cocci-dian parasite of carp.