The filtering rates of Calanus helgolandicus were measured on synchronously growing populations of the diatom Ditylum brightwellii, by following changes in cell numbers and also by measuring the ingestion of 14C labeled cells at selected intervals during 2‐day periods. The growth of the diatoms was adjusted to be one cell division per day by selecting appropriate photoperiods and light intensities. When the alga was synchronized, cell division took place in the dark.Filtering rates were calculated separately for animals grazing on single cells and on “paired” daughter cells (before separation after division). The volume of paired cells was 1.5 times that of the singles. There was no significant difference in the filtering of single cells in the light or dark part of the cycle. Filtering on paired cells was significantly higher in the dark or at those times when high percentages of pairs were present. Between 0 and 20% pairs, the animals filtered at a constant rate of 50 ml copepod−1 day−1. Above 40% pairs, there was a maximum filtering rate of approximately 170 ml copepod−1 day−1. Between 20–40% pairs, the filtering rate increased proportionally to the percentage of pairs, and ingestion of paired cells increased exponentially in relation to their concentration.It is suggested that C. helgolandicus actively hunted and selected the paired cells and passively filtered the singles. In addition, the data support the idea that there is no innate diurnal feeding rhythm in these copepods.