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The effect of a heat shock pre-treatment (45 °C for 5 min) on the survival and division of rice (Oryza sativa cv. Taipei 309) cell suspension culture protoplasts was investigated. Heat shock treated protoplasts exhibited an effective doubling in both the frequency of plated protoplasts dividing after 10-14 days (45 %) and in sustained division (plating efficiency 1.1 % after 30 days) compared with untreated protoplasts. The optimum plating density for sustained division was 3.0 × 10⁵/ml. Colonies were obtained only from protoplasts subjected to the heat shock when cultured at a reduced density of 1.0 × 10⁵/ml. This is the first report of enhanced protoplast division following an elevated temperature pretreatment. The implications of these findings and some possible explanations for the observations are discussed.