A maternally transmitted, noninfectious disease known as the Cayuga syndrome caused 100% mortality in larval offspring of wild-caught landlocked Atlantic salmonSalmo salarfrom several of New York's Finger Lakes. Survival of lake troutSalvelinus namaycushfrom Lakes Erie and Ontario was also impaired, but not until yolk absorption was nearly complete; moreover, mortality was greatly reduced relative to that of the salmon (range: 5–87%). Tissue concentrations of thiamine hydrochloride were severely reduced in these salmonid fish relative to unaffected control stocks. Afflicted Atlantic salmon treated with thiamine by yolk-sac injection or by bath immersion recovered completely from the Cayuga syndrome, as evidenced by the quantified reversal of abnormal swimming behaviors only 2 d after treatment and by the excellent survival (>95%) of the treated Atlantic salmon through 1.5 months of feeding. These data represent the first evidence of a vitamin deficiency causing the complete reproductive failure of an animal population in nature. These lethal vitamin deficiencies are presumably caused by a diet of alewivesAlosa pseudoharengus, nonnative forage fishes of the herring family that exhibit high thiaminase activity.