One hundred and twenty-one specimens of Calanus finmarchicus were anæsthetised in a dilute solution of Urethane (2·5 grms. in 100 c.c. of seawater) and the time taken by each to sink through a measured distance determined. The length of each specimen, in millimetres, was noted, and the relationship between sinking time and length of the individual was determined. The coefficient of correlation, r, was found to be —0·63.Apstein (1910, p. 23), working with preserved material, found that the time taken by adult Calanus was less than that taken by young. He does not give actual length measurements. Eyden (1923, p. 53) showed that for anaesthetised specimens of Daphnia pulex, the average rate of fall was approximately twice as great in specimens of length 2·5–3·0 mm. as in smaller ones, whose length varied from 1·5–2·0 mm.The position in which specimens sank was conditioned by the manner in which the antennæ were held. It was found, however, that this had not as great an influence on the sinking rate as might have been expected.Of the 121 specimens examined, 21 were in Stage IV, 41 in Stage V; 59 were adult, of which 54 were females and only 5 were males.