From August in 1978 to August in 1980, a continuous field investigation was made on Alella macrotrachelus infestation in black sea-bream (Acanthopagrus schlegeli) cultured in a private farm in the Inland Sea of Japan. 1) A. macrotrachelus was observed on the gills of the fish from the farm throughout the year.The parasite was also common in some other farms, but few in which were situated near the mouth of rivers. 2) The average number of the female parasites on a fish began to increase immediately after the start of rearing (August), and reached the first peak (ca. 15 worms/fish) in winter (January or February). It attained the second peak in the next spring, then decreased considerablly in late summer, when some of the cultured population began to be harvested. 3) The most dangerous stage of this parasitic disease seemed to be in the first winter. 4) From histopathological observations, it was found that the female parasite took the epithelial cells of gill lamellae and erythrocytes. The hyperplasia of the epithelial cell layer, edema and hemorrhage in the gill lamellae were also caused by the parasite.