Traces of cadmium have been determined by neutron activation analysis in a suite of rocks from the Skaergaard intrusion, East Greenland, and in the separated minerals from one of them. The original parent basic magma contained 0·13 p.p.m. Cd, and progressive fractional crystallization led slowly to a twofold increase in the ferrogabbros and a fourfold increase in the granophyres derived largely from the ultimate residual magmatic fractions. Cd does not appear to enter immiscible copper and iron sulphide droplets in the magma in preference to silicates and oxides, neither does it show any marked preference among the latter. Despite similar ionic radius and charge, Cd 2+ does not follow Ca 2+ in plagioclase and apatite, but is slightly concentrated in iron-rich pyroxene, olivine and titaniferous magnetite. Most of the Cd becomes concentrated in the residual magmatic liquids. The values for Cd found by the neutron activation method in the standard rocks G-1 and W-1 were 0·06 p.p.m. and 0·33 p.p.m., respectively.