Summary An early phase of encystment of the ciliate, Histriculus cavicola is subdivided into 5 stages, stages A to E. Resorption of feeding organelles, undulating membranes (UM) and the adorai zone of membranelles (AZM) start from stage B. In stage E cells, resorption of UM is completed and that of the AZM reaches to the level of anterior end of left marginal cirral row from its posterior end. When stage A cells are transferred in culture medium with or without food organisms, or in normal or 10 times concentrated Osterhout's solutions with food, most cells do not encyst. More than half of stage C cells do not encyst at 20°C after transfer in culture medium. All stage E cells encyst, irrespective of the medium or of the presence or -the absence of food into which they are transferred. About half of stage A cells and all stage B to E cells contain ectocyst precursors. Our results indicate that initiation of ectocyst precursor formation does not necessarily direct the cells to encyst but cells in stage E are determined to encyst.