Light and electron-microscopic observations of the blood cells (haemocytes) of the stick insect Clitummus extradentatus in vitro showed that two morphologically distinct cell types, the cystocytes and granular cells are involved in haemolymph coagulation. Both these cell types contain a variable number of electron-dense granules which upon release cause progressive coagulation and precipitation of the haemolymph. In the cystocytes this release is extremely rapid (30-60s) while in the granular cells it only occurs after 1-2 min in vitro. The role of these cells in haemostasis is discussed together with the possible significance of the involvement of two cell types in the coagulation process.