On the basis of patterns of haemocyte ultrastructure and functions at preimago and imago stages of Metrioptera roeselii, secretory cells of granulocyte type were recognized in the haemolymph. The development of granulocytes was traced starting from their formation up to cell death and destruction. The haemocytes develop as "dark" and "light" cells, differing in their functional activity, although their ultrastructure is similar. In the cytoplasm of granulocytes, granules of both mitochondrial and nuclear origin were detected. Simultaneously two processes occur in the cells--the accumulation and discharge of granules.