This study was undertaken to examine the influence of temperature on granulocytes in the nymph of american cockroach, P. americana, using light microscope and scanning electron microscope. The nymph of american cockroach, P. americana, which was bred at 28$^{circ}C$, was treated at temperatures ranging from 20$^{circ}C$ to 0$^{circ}C$, with the interval of 5 degrees between the temperatures 20$^{circ}C$, 15$^{circ}C$, 10$^{circ}C$, 5$^{circ}C$ and 0$^{circ}C$, and it was examined in 12hrs, 24hrs, 36hrs and 48hrs at each temperature. Haemolymph was always taken from antennae. Shapes of the granulocytes turned out to be oval or round at 20$^{circ}C$, which are almost identical with the shapes at 28$^{circ}C$, The number of amoeboid granulocytes began to increase at 15$^{circ}C$ in 12hrs, and at 10$^{circ}C$ in 12hrs most of the granulocytes became amoeboid. Beginning fusiform granulocytes appeared at 10$^{circ}C$ in 24hrs and most of granulocytes were fusiform at 5$^{circ}C$ in 36hrs. Cytoplasm decreased markedly and granulocytes with elongate nuclei appeared at 5$^{circ}C$ in 48hrs. The granulocytes with elongate nuclei were observed easily at 0$^{circ}C$ in 24hrs. These observations demonstrated that haemocytes in the nymph of american cockroach, P. americana, are markedly influenced by low temperature (15$^{circ}C$∼0$^{circ}C$); that is, the size of haemocyte decreases and the shape of the granulocytes changes.