The mortality of ichthyoplankton entrained through a scale model of a towboat propeller was evaluated in a large (>2 million L) circulating water channel. Five species of fish (larval shovelnose sturgeon Scaphirhynchus platorynchus, larval lake sturgeon Acipenser fulvescens, the larvae and eggs of paddlefish Polodon spathula, larval blue sucker Cycleptus elongatus, and juvenile common carp Cyprinus carpio) were tested. Replicate group, of individuals were subjected to one or more shear stress levels (634, 1,613, 3,058, and 4,743 dynes/cm(2) [I dyne = the force that would give a free mass of I g an acceleration of 1 cm/s(2)]). Mortality was a linear function of shear stress for all species and life stages. Paddlefish eggs and common carp juveniles were more resistant to shear stress than all larval fish. Smaller larvae (take sturgeon and blue suckers) experienced higher mortalities (>75%) than larger larvae (shovelnose sturgeon and paddlefish). The conditional mortality (i.e., treatment less control mortality) of blue suckers, the smallest larvae evaluated during the study, was less than 40% due to the high mortality of control groups from handling stress. Delayed mortality was monitored over a 1-3 h period and was observed for all larval species, particularly at higher shear stress levels, but not for common carp juveniles and paddlefish eggs. The probability of blade contact was approximately 10%, but only a few juvenile common carp displayed blade-type injuries. Shear stress created from propellerjet velocities in navigable rivers can exceed 5,000 dynes/cm2 and is probably the primary force contributing to the mortality of ichthyoplankton entrained during vessel passage, the magnitude of which is dependent on individual size