Abstract.An ultrastructural study of the cellular response of plaice toRhipidocotyle johnstoneinom. nov. Matthews, 1968, revealed a chronic inflammatory lesion containing cells mainly of the mononuclear phagocyte series. The ‘granuloma’ included monocytes, macrophages, epithelioid cells and multinucleated cells with peripheral fibrocytes. The presence of small numbers of lymphocytes and plasma cells in the mature granuloma suggests a cell‐mediated response. The parasite becomes adult within two months and survives for approximately 10 months, the capsules becoming increasingly fibrotic. Heavy infections in young fish cause fin erosion, haemorrhage and probably impair muscle function, increasing the risk of pred