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Executive Summary Goal of this study was to provide information required to establish guidelines for minimizing detrimental effects of electrofishing in Recovery Program studies, specifically on use of electrofishing to capture adult razorback sucker Xyrauchen texanus. Objective was to determine if exposure of ripe razorback sucker to presently used or potentially less harmful electrofishing fields would injure fish and(or) adversely impact the viability of gametes, thereby reducing egg hatching success. The objective was achieved. In the laboratory, we assessed direct effects of electroshock by two types of square-wave pulsed DC in homogeneous fields of 1.0 peak V/cm on ripe razorback sucker and evaluated subsequent egg hatching success. Four males and four females were exposed for 10 s to a simple, 60-Hz (24% duty cycle) current, and four males and three females were subjected for 10 s to a complex-pulse current (CPS™) of three 240-Hz, 2.6-ms pulses delivered at 15 Hz (12% duty cycle). All shocked fish expelled some gametes during treatment. No external hemorrhages were observed, but X-ray and necropsy examinations revealed injuries associated with the spinal column in two males and two females subjected to the 60-Hz current and one female exposed to CPS; no injuries were observed in the four control fish (two males and two females). Mean percent egg hatch for fish shocked by either current was significantly lower (P ~ 0.05) than that for control fish. Difference in hatching success between treatment currents was not significant. Electrofishing could adversely impact razorback sucker populations by injuring adults and reducing their successful reproduction. We recommend that the need for electrofishing over active razorback sucker spawning areas should be carefully evaluated. Key Words.-Razorback sucker, electrofishing, injury, reproduction, eggs, Green River. 4