The blood circulatory system and the circulatory dynamics of the spiny lobster Panulirus interruptus (Randall) have been examined in detail. A mean absolute blood pressure of 36.5 cm-H2O and a mean pulse pressure of 27 cm-H2O, show this species to have the highest pressure-driven circulation yet measured in decapod crustaceans. Mean blood velocities have been directly measured in intact animals for the first time. These velocities range from over 10 cm/sec in the large arteries to less than 1.0 cm/sec in the sinuses. Total blood volume is variable but averages near 30% of the wet weight, while heart volume averages 0.3% of the wet weight. The blood velocity and volume data are used to compute circulatory parameters such as cardiac output, stroke volume and turnover time. Comparisons with other animals suggest that the so-called “open-system” characteristic of crustaceans may be much more effective than previously supposed.