A new method of releasing cellular soluble organic matter from microalgae is reported. This method, which could be used to help characterize the production and taxonomic composition of a phytoplankton bloom, was developed in the laboratory to determine the transition temperature, Tm, for planktonic algae. The Tm of these algae is defined as the temperature at which half the cellular soluble carbon is found extracellularly under standard conditions, by analogy with the definition of Tm in bacteriology. Values of Tm in the range of 23–46·5 °C are reported for six species of Bacillariophyceae (diatoms) and two species of Prymnesiophyceae. Species-specific Tm values are highly reproducible and are regarded as an intrinsic property of the species and its physiological races. Possible field uses of the method are discussed.