Pleurotus pulmonarius fruit body primordia (FBP) formed on potato extract agar (PEA) and wheat straw-based media (WS) were studied. When grown on PEA, FBP hyphae had a large number of vacuoles and less stainable cytoplasmic material. In contrast, when grown on WS, there was abundant cytoplasmic material and fewer vacuoles. FBP grown on WS had a more well-defined mushroom shape than those grown on PEA. The average hyphal diameters on PEA and WS were 4.15 and 3.52 micro m, respectively; the average hyphal lengths were 11.75 and 11.80 micro m, respectively. The control mechanisms regulating formation of the normal shape and size of FBP might be more dependent on the amount of cytoplasmic material than on the physical volume of the hyphal compartment. Since the ultrastructure of the hyphal compartment, which makes up the substance of the FBP, depends on the substrate in which the FBP has been grown, the composition of the substrate may be important for manipulating the nutritive and organoleptic characteristics of the mushroom crop.