Summary: The compositions of apatite crystals from seven Skaergaard rocks were obtained by electron-probe microanalysis. These span the range of occurrence from the base to the top of the exposed zones of the layered series. Apatite occurs as an intercumulus phase in the lower zones, but becomes a cumulus phase at a structural height of 1850 m (c. 98 % crystallized), defining the base of Upper Zone b. All apatite analyses show a high F:Cl ratio. There is a slight but significant increase in F and decrease in Cl when the apatite becomes a cumulus phase, the F/Cl values changing from < 10 to > 30. Variations in F:Cl:OH are attributed to differential volatile migration from trapped intercumulus liquid sites. The apatite data provide new support for ferrodiorite-granophyre liquid immiscibility.