Measurements have been made of the quantity and type of food assimilated by adult female Calanus helgolandicus (Claus) during the summer months.

Animals kept in the dark at 10° C under a continuous flow of ‘outside’ Plymouth sea water (0·95–2·50 mg insoluble organic material/1.) removed 26·0–66·5 μg/day/Calanus, of which 74–91% was actually digested.

Volumes of sea water filtered daily varied between 10·0 and 36·0 ml./ animal, with an average of 21·5.

The animals preferentially selected a diet of high organic content from the particulate food available, digesting average daily quantities of 18·1 μg carbohydrate, 6·5 μg lipid and 2·7 μg protein.

The average amount of food digested daily accounted for 25·3% of the dry weight and was equivalent to an average respiration rate of 26·5 μl. O2/ animal/day, which adequately accounts for the highest values reported by others for C. finmarchicus (Gunnerus) kept under similar conditions of light and temperature.

These results, which provide direct evidence that Calanus obtains the bulk of its food from particulate material present in the sea, have been discussed with especial reference to Pütter's hypothesis.