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3 Recent declines in populations of shortnose suckers Chasmistes brevirostris and Lost River suckers Deltistes luxatus, native to the Upper Klamath River system in California and Oregon, resulted in their addition to the federal endangered species list in 1988 (Williams 1988). Their declines have been attributed to a wide array of factors, including the presumed negative impacts of exotic species (USFWS 1992). Hypothesized negative impacts of non-native fish on endangered suckers include (among other things) predation, competition for food and space, and introduction of exotic diseases or parasites, although no research has assessed these hypotheses. The most recently established exotic fish in the Upper Klamath River system is the fathead minnow Pimephales promelas, which probably became established in Upper Klamath Lake between 1974 and 1979 (Andreasen 1975; Ziller 1991). Fathead minnows rapidly increased in numbers in Upper Klamath Lake thereafter, they comprised 18 % of the catch by number in a trap net in 1983 (Ziller 1991). In 1992, fathead minnows numerically dominated fish caught in trap nets set in Upper Klamath and Agency Lakes (directly connected to Upper Klamath Lake) by a wide margin (Logan and Markle 1993). High densities of adult fathead minnows occupy the same shoreline nursery habitats as larval suckers in Upper Klamath Lake (Buettner and Scoppettone 1990), prompting concern about their potential predatory impacts. While food habits of fathead minnows in Upper Klamath Lake are unknown, elsewhere their diets are reported to include algae, aquatic insect larvae, zooplanltton, detritus, and mud (Carlander 1969; Scott and Crossman 1973; Held and Peterka 1974). The only record I found of piscivory was in Franzin and Harbicht (1992), who found walleye Stizostedion vitreum larvae in stomachs of fathead minnows captured in trap nets. However, they were uncertain whether the predation occurred befo captum r resulted ntrating redato s Jae+ ,, and prey in the trap net. -CrVCny In a a ac?e.u? miz pre,xperimental trial for this study iaAvW*h- five fathead minnows ate d- A L-t . 44"4 ct 39 of 40 Lost River sucker larvae.shewed that fathead minnows were willing and able to consume sucker larvae. Th s demonstration of piscivory by fathead minnows coupled with their high densities in nursery habitats of endangered suckers in Upper Klamath Lake prompted study of the predatory interactions of fathead minnows with sucker larvae under laboratory conditions. In this study I assessed predation rates of fathead minnows on sucker larvae in tanks as influenced by water depth, the presence and absence of cover, the presence of alternate prey, and fathead, minnow mouth size relative to body size of sucker larvae.