The cellular responses of Crenimugil labrosus and Pleuronectes platessa to tissue parasites have been investigated using metacercariae of Cryptocotyle lingua and Rhipidocotyle, johnstonei, and the myxosporidjan Myxobolus exiguus, The study was based on fish naturally infected with the parasites, and in addition experimental infections of C. lingua ' were established in grey mullet. The most intense response was induced by R. -johnstonel in plaice, and was composed mainly of cells of the macrophage series, which included epithelioid and giant cells. Fibroblasts and collagen deposition formed the periphery of the lesion which was Interpreted as a granuloma. Eggs released by this progenetic metacercaria did not produce separate granulomata. Fibrotic encapsulation and associated melanin deposition formed the host response to the encysted metacercariae of C. lingua in naturally infected plaice and mullet. infections of mullet produced only a fibrotic capsule. Sub-dermal cysts of M. exiguus produced a slight fibrosis and associated melanin deposition. Other sites produced little response. slight hyperplasia associated with interldmellar cysts. Haematological studies on mullet and turbot formed a basis for the Interpretation of the cellular responses to the parasites. Lymphocytes, monocytes, thrombocytes and granulocytes were identified from both species, and additionally plasma cells from mullet. Phagocytic capacity of circulating monocytes was demonstrated In turb-ot. Experimental infections of C. lingua in mullet were used additionally to study the formation of the parasitic cyst wall and related changes in the tegument during the metamorphosis from the cercarial to metacercarial stage. The two acellular layers of the parasitic cyst were formed from secretion bodies from the tegument and cystogenic gland cells. Breakdown of the tegument of the early metacercaria preceded replacement by the fully formed metacercarial ýegumentg which was initially microvillous and later contained secretion bodies. Variation in the strqcture of the plasmodial wall of bl. exiguus occurred with site of development, although sporogenesia was similar to other species. A hyperparasitic microsporidian Nosemoides M. was found in some plasmodia.