The vertical distribution of plankton organisms and their eggs depends largely on their surface area and density (Eyden, 1923). Gross & Raymont (1942) observed that the eggs of Calanus finmarchicus had a density (1.045–1.049 g/cc) greater than that of sea water (1.0235–1.025 g/cc), and that they sank at the rate of approximately 2.5 cm/min at 13°C. These workers pointed out that since the embryo takes 24 h to complete development, the eggs will sink in still water to a depth of 36 m before hatching and yet the data available, although not extensive, suggests that both eggs and nauplii are found most abundantly nearer the surface than this (Nicholls, 1933; Kraefft, 1910). If these observations are correct, then in addition to density and viscosity, there must be a third factor affecting the distribution of these eggs. Unfortunately, Gross & Raymont did not consider the possible effect of viscosity in their method for determining density, and they themselves commented on the disturbing degree of variation in some of their results. Their measurements of the rate of sinking of the eggs in sea water also showed considerable variation. It seemed worth while, therefore, to measure the density of these eggs using the density gradient technique of Linderstrøm-Lang (1937), a method which enables the individual and precise measurement of the density of small living objects, independent of their shape, i.e. surface area. This method was being used in work on Psammechinus eggs at the Marine Station, Millport, and, at the suggestion of Dr A. P. Orr, the following measurements were made on the eggs of Calanus finmarchicus.