Abstract: Mast cells were identif ied first in lymphoid organs including thymus, head kidney and the splenic sinus of grass f ish ( Ctenopharyngodon idella ) histochemically and morphologically . It seemed that the staining properties of grass f ish mast cells were similar to mammalian mucosal mast cells in some aspects. Carnoy.. s fluid and Alcian blue proved to be the good f ixative and excellent dye for grass fish mast cell, respectively , but neutral buffered formalin ( NBF) completely blocked the staining of the cells. It was noticed that some other histochemical methods previously used to detect mast cells, or so called eosinophilic granule cell ( EGC ) in teleo stean fish, failed to identify grass f ish mast cells in present study. Under electron microscopy a great deal of characteristic cytoplasmic granules were observed in the grass f ish mast cells. The granules contained amorpholous granular matrix but electron density of the matrix from low to high. Special substructures were not observed in the cytogranules.