Organ distribution of leucocyte subpopulations was examined in carp(Cyprinus carpio)development by flow cytometric analysis using monoclonal antibodies raised against: carp immunoglobulins, early thymocytes, thrombocytes, and monocytes/macrophages. In thymus, a high percentage of early thymocytes was found. In head kidney, Ig‐bearing cells first appeared more numerous than in other organs. In spleen only early thymocytes and thrombocytes are relevant. In gut, early thymocytes are detected in the first week, but they significantly decrease during ontogeny. The percentage of Ig‐bearing cells remains low throughout development. In peripheral blood leucocytes of 1‐week‐old carp, early T cells are the only detectable leucocyte subpopulation. From 7 days onwards, the percentage of monocyte/macrophages increased; thereafter, thrombocytes and B cells became detectable. This study provides new indications on the major sites of lymphopoiesis and thrombopoiesis in carp.