Abstract: Cellular alterations of gills of freshwater prawn, Macrobrachium rosenbergii to three temperatures (25, 30 and 35°C) was assessed by means of Transmission Electron Microscopy (TEM). Acclimation of adult prawns (Macrobrachium rosenbergii) was carried out at one degree/day over and below ambient water temperature (30°C) to reach test temperatures (25, 30 and 35°C) and maintained for 30 days. Acclimated prawns were subjected to a constant rate of increase or decrease (0.3°C/min) until critical thermal maxima (CTMax) and critical thermal minima (CTMin) were reached. Dissected out gills were processed for TEM, both at the end of acclimation period and at critical thermal limits. Results indicated that at 25 and 30°C, gill tissues appeared normal. However, severe destruction of gill lamellae was observed at 35°C. At 25 and 30°C, primary lamellae showed active with euchromatin appeared dispersed in the matrix. At 35°C, severe degeneration of mitochondriae with loss of cristae, irregular nuclear membrane and margination of chromatin were evident. Gill tissues at CTMax showed severely damaged lamellae, increased number of lysosomes and karyorhexis, destruction of mitochondria and nuclear membrane, were observed. At CTMin, gill tissues exhibited little visible cellular damage as compared to CTMax. Our prima-facie report indicates M. rosenbergii demonstrates acclimatory responses to temperature. However, cellular integrity may be affected due to long-term exposure to higher temperatures (35°C).