The Lost River sucker (Deltistes luxatus) is a federally listed, endangered species inhabiting the hypereutrophic waters of Upper Klamath Lake in southern Oregon, USA. High pH (> or =10) and elevated ammonia concentrations (> or =1 mg NH(3)-N/L) often occur during blooms of cyanobacteria (Aphanizomenon flos-aquae) in the lake, with major fish kills sometimes following a mid- or late-summer "crash" of the cyanobacterial population. Previous histopathology analyses and bacterial sampling indicated that infections of the pathogenic bacterium Flavobacterium columnare might contribute to the fish kills. We hypothesized that prior exposure to adverse water quality conditions increases the susceptibility of Lost River suckers to F. columnare infections. To test this, we exposed juvenile Lost River suckers to four sublethal ammonia concentrations at pH 9.4 for 62 d. On day 31, fish in half of the aquaria were exposed to F. columnare. As expected, survival of the Lost River suckers decreased in aquaria inoculated with F. columnare. Ninety-four percent of the fish that died were infected by F. columnare in the gills, kidney, or skin, whereas none of the survivors or unexposed control fish was infected. However, contrary to our hypothesis, survival of the fish exposed to F. columnare increased significantly (p < 0.05) as unionized ammonia concentrations increased. Our results suggest that complex interactions can complicate prediction of the responses of fish to concurrent chemical stressors and bacterial pathogens.