Helminth parasites are one of the major constraints in freshwater aquaculture, particularly in hatchery and nursery systems where fish are reared under intensive conditions. The present study was conducted to investigate the prevalence of helminth infections in cultured fish from hatcheries and nurseries of Supaul district in the Koshi basin. Fish samples were collected from selected hatcheries and nursery ponds and examined for parasitic infestation. Trematodes, cestodes, and nematodes were observed in different organs of infected fishes. The prevalence of infection was higher in nursery ponds compared to hatcheries. Infected fishes showed reduced growth, abnormal swimming behavior, and tissue damage. The study indicates that helminth infections adversely affect fish health and seed quality. Proper pond management and regular monitoring are essential to control helminth infestation and improve aquaculture productivity. Adoption of preventive measures such as periodic pond drying, liming, and removal of excess organic matter can significantly reduce parasite load in culture systems. Maintenance of optimum water quality parameters and regular exchange of pond water also help in minimizing the spread of helminths. Control of intermediate hosts like snails and aquatic insects is equally important for breaking the life cycle of parasites. Stocking density should be maintained at recommended levels to avoid overcrowding and stress in fishes. In addition, routine health examination of fish seed and timely application of prophylactic treatments may prevent severe outbreaks. Implementation of these management practices will enhance survival rate, improve growth performance, and ensure production of healthy fish seed in hatchery and nursery systems of the Koshi basin.