1. The seasonal production of zoöplankton, particularly of Calanus finmarchicus, was investigated by means of scrim nets and a plankton pump at five stations in the vicinity of Woods Hole during the summer of 1935 and at, one offshore station throughout the ensuing year. Collections of phytoplankton and of nannoplankton and measurements of temperature, salinity, phosphates, nitrates, and illumination were carried out at the same time. 2. The zoöplankton consisted largely of copepods, but medusæ, sagittæand, at the stations near shore, larvæ oof both benthonic and planktonic forms occurred irregularly. Sagitta elegans exhibited four main breeding periods during the year. 3. The numerical analysis of the Calanus population revealed the presence of nauplii in significant numbers in March, April, and May, and of early copepodites in April-May and June-July. Stage V was found at all seasons. Adult specimens were relatively scarce at all times, but most abundant in May and June. The species as a whole was reduced to small numbers during the autumn and winter. 4. The pump catches and the net hauls agree in indicating that a short-lived generation of Calanus occurs during the spring and that this is followed by a long-lived generation. The latter has its origin in the early summer, passes through the autumn and winter as Stage V, and matures early the following spring to give rise to the next short-lived generation. 5. The measurement of the cephalothorax length of 25 specimens in each stage older than copepodid Stage II showed that the average lengths of the various groups did not fluctuate widely during the year except in the case of the adults which exhibited a sudden drop in size in May. 6. The spawning periods of Calanus did not occur at times of diatom maxima, and therefore the two phenomena are not directly related in the present case. In regard to the nourishment of this copepod we must conclude (a) that the small number of diatoms always present as a minimum is sufficient, or (b) that the animals build up sufficient reserve on occasions of small local flowerings of diatoms, which occur at intervals of a week or two, to tide them over the intervening periods, or (c) that "flagellates," which were found continually in large numbers, and other types of nannoplankton are important as a food source.