Summary: Rhythmic layering in the Skaergaard intrusion shows variation in crystal size and in modal proportions of primocrysts with structural height. These changes are ascribed to variations in nucleation rates of primocrysts. The nucleation theory requires that crystallization occurs under supercooled conditions, and that the crystallization of one primocryst phase may change the composition of the magma in such a manner that the nucleation rates of other primocrysts are influenced. Both these constraints appear to have been fulfilled for the Skaergaard intrusion.