Microscopical studies of Leucothrix sp. and its histological effect on the gill tissue of Penaeus japanicus have been made. Infection of Leucothrix sp. results in color change of the gills of the shrimp which is mainly due to the brown or black pigments of the filamentous bacteria. Higher degree of the infections makes necrosis and serious histological changes of the gill tissue and this infection affects the respiratory activity of the shrimp.