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Introduction The razorback sucker Xyrauchen texanus is a federally endangered species endemic to the Colorado River basin (USFWS 1991). This fish was once widespread and common throughout larger rivers of the basin, but only disjunct populations and scattered individuals exist today (Minckley et al. 1991). The largest extant riverine population of razorback sucker occurs in the middle Green River system of northeastern Utah and northwestern Colorado, but number of adults is low and recruitment is minimal (Tyus 1987; Lanigan and Tyus 1989; Modde et al. in press). Electrofishing of spawning aggregations is an effective method for capturing adult razorback sucker in rivers (Tyus 1987; Tyus and Karp 1990), but concerns exist within the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin over potential harmful impacts to spawning fish, their gametes, and early offspring. This study was part of investigations to address those concerns and provide information for establishing guidelines on use of electrofishing to monitor endangered fishes in the Colorado River basin. Our objective was to determine if electroshock by two types of pulsed DC affected captive ripe razorback sucker and subsequent egg hatching success. Methods Ten ripe males and 10 females in near-ripe condition were allotted for this study from razorback sucker brood stock maintained at Dexter National Fish Hatchery and Technology Center, New Mexico. Males were 499-551 mm total length (mean, 526) and weighed 1.36-1.80 kg (mean, 1.63). Females were 551-599 mm total length (mean, 575) and weighed 1.93-2.61 kg (mean, 2.28). Fish were transported to U.S. Fish and Wildlife Service facilities in Grand Junction, Colorado, for final maturation of eggs, electroshocking, and artificial fertilization of eggs. Males and females were separated and held indoors in hauling tanks with aerated water at 17-20oC; the largest female was injured during removal from the hatchery raceway and died prior to treatment, about 16 h after transport. Ovulation was induced by intramuscular injections of 220 units human chorionic gonadotropin/kg body weight at 24-h intervals (Hamman 1985) beginning on the day of transport. Six of the remaining nine females began ovulating after receiving two injections, and the other three began ovulating after receiving a third injection. Electroshocking of fish started when all females were ovulating. Electric systems were powered by a Honda@ model EG5000X, 5000-W, 240-V generator. Alternating current from the generator was transformed to square-wave 5