The descriptions of the seasonal cycles in abundance of the major holozooplankton are made based on a 2 year period of net samples taken in the innermost part of Onagawa Bay, northeast mainland of Japan. Acartia clausi was the dominant copepod during most of the year, particularly abundant in the summer and fall, followed by Paracalanus parvus. Eurytemora pacifica and Pseudocalanus minutus were abuliclant in the winter and spring. Oithona similis was constantly present throughout the year, with peak abundance in the spring and summer. Noctiluca miliaris, Hydrozoa, Chaetognatha and Appendicularia were generally abundant in the summer and fall. Each of the 5 species of Cladocera displayed remarkable seasonal fluctuations in population abundance. The narrower range of annual temperature variation of Onagawa Bay allows relatively constant presence in the plankton for many species of organisms.