Five types of haemocytes were identified in Poecilocerus pictus (Fab.) and their ultrastructural details and differential counts were studied. The types identified are the prohaemocyte, plasmatocyte, granular haemocyte, oenocytoid and coagulocyte. In response to the injection of yeast cells into the haemocoel, changes in haemocyte morphology and differential counts occurred. The predominant cell involved in phagocytosis is the plasmatocyte, followed by granular haemocyte.