Abstract 1. 1.|Ultrastructural organization of Stylonychia mytilus (a ciliated protozoan) cells is studied in response to hyperthermia treatment (30°C). 2. 2.|Cytoplasmic organization is altered as compared to the control cells grown at 23°C. The channel system expands and thereby restricts the membrane-bound cytoplasm. Tubular bodies of the channel system are scarce. Vesicles of the rough endoplasmic reticulum show dilation and there is persistent development of smooth endoplasmic reticulum. 3. 3.|Membrane-bound vesicles containing osmiophilic bodies appear in these cells. Macronuclear chromatin gets dispersed, shows a reticular pattern of dense chromatin an the appearance of microfilaments (4–5 nm). Nucleoli are hypertrophied and develop a fine fibrillar are in the centre with a fibrillar and granular zones at the rim. 4. 4.|The study reveals the subcellular adaptations of heat treated Stylonychia cells which otherwise exhibit normal external morphology and growth characteristics.