Main channel spawning of the flannelmouth sucker Catostomus latipinnis was investigated during the spring of 1997 and 1998 in the tailwater of Glen Canyon Dam, Colorado River, Arizona. Predominantly female groups of flannelmouth suckers aggregated in warm water areas prior to the arrival of suckers on the spawning bar. Densities of suckers in aggregations decreased in early to mid-May as fish arrived on the spawning bar. Peak spawning activity occurred two to three weeks after suckers arrived on the spawning bars. Spawning fish remained on the bar for approximately two months. Dam releases varied between 1997 and 1998. Low flows during the weekends in the spring 1998 left the spawning bar above the waterline, desiccating spawning areas. No larval fish were captured near the spawning bar in 1997 or 1998. Water temperatures within the tailwater may exceed lower lethal limits of egg survival for flannelmouth sucker. Suckers originally marked at known spawning areas as far as 120 miles down stream were recaptured on the tailwater spawning bar. It is likely that recruitment from successful spawning populations down river support the resident population in the Glen Canyon Dam tailwater. The student finished an MS thesis on this project in 2003 and is now writing up results for publication.