Little is known about the characteristics of hemolymph (blood) flow in animals with open circulatory systems. We measured cardiac output and blood flow to specific tissues in the black abalone, Haliotis cracherodii, a gastropod mollusk. The use of thermodilution allowed us to make repeated measurements of cardiac output and cardiac stroke volume over relatively short time intervals (5-10 heart beats) in resting, unrestrained abalone while disturbing the animals minimally. Anatomical studies of the abalone circulation showed that the arterial system terminated in small diameter (approaching 10-20 micron in some cases) lacunar tissue spaces. Because of this, we used radioactive microspheres (which must be trapped in the tissue vasculature) to measure blood flow rates to selected tissues. The major findings of our study were that 1) cardiac output in the black abalone ranged from about 100 to 150 ml X kg-1 X min-1, and was highly correlated with body size; 2) weight-specific cardiac stroke volume was about 5 ml X kg-1, considerably larger than that of a mammal; 3) tissue blood flow rates ranged from 10 ml X 100 g-1 X min-1 (foot muscle) to 80 ml X 100 g-1 X min-1 (nephridial tissue), similar to typical tissue blood flow rates in mammals. Our data suggest that the blood in the abalone is directed to the tissues not in proportion to percent total body weight the tissues represent (as might be expected in an open vascular system), but apparently in proportion to tissue metabolic rate.