Membranous glomerulonephritis caused in Barbus graellsi by myxosporidian infections have been studied by electron microscopy and immunoelectron microscopy techniques. This study indicates that Myxosporidian infection produces a chronic severe aggression. Spores reach the spleen, the kidney and the liver, where they are trapped and phagocyted by Melano Macrophage Centres. Consequently, the commencement of a immunological response to myxosporidian is evident. Our results show the presence of immunodeposits in the basement membrane of the glomeruli, suggesting that they might initiate glomerulonephritis. The lesion was markedly similar to immune complex-mediated glomerulonephritis disease in higher vertebrates.