1. Survivorship curves and their parameters are determined at different temperatures for the marine 'Cercaria caribbea' LXXI (Cable). 2. A relationship is demonstrated between Mean Expected Life Span and half life of the populations (t₀.₅) and it is argued that t₀.₅ is a good index of glycogen utilization rate. 3. Q₁₀ values for t₀.₅ computed on this basis were 5.8 between 10 and 20°C, 3.1 between 30 and 40°C and ~- 1 between 20 and 30°C. 4. The zone of thermostability corresponds with normal environmental temperatures for the cercariae and probably serves to maximize infectivity under prevailing environmental conditions.