The cell adhesion molecule L1 plays crucial roles in axon tract development. In vitro, L1 presented as a culture substrate stimulates axon elongation by binding to L1 expressed on the growth cone. In migrating growth cones, L1 is endocytosed via the AP-2/clathrin-mediated pathway at the central domain, followed by anterograde vesicular transport and recycling to the plasma membrane of the leading front. It has previously been shown that this endocytic trafficking of L1 is critical for axon elongation (Kamiguchi and Yoshihara [2001] J. Neurosci. 21:9194-9203). Adjacent to the AP-2 recognition site, the L1 cytoplasmic domain has a cluster of acidic amino acids containing Ser1181 that can be phosphorylated by casein kinase II (CKII; Wong et al. [1996a] J. Neurochem. 66:779-786). In this paper, we demonstrate that Ser1181 phosphorylation by CKII is implicated in both normal endocytic trafficking of L1 and L1-stimulated axon growth. Whereas L1 is sorted into transferrin-positive endosomes after endocytosis, pharmacological inhibition of CKII caused some population of L1 to be internalized into transferrin-negative compartments. Single-amino-acid mutations at Ser1181, which either prevent or mimic phosphorylation by CKII, caused similar missorting of internalized L1. Furthermore, dorsal root ganglion neurons that had been treated with a CKII inhibitor or transfected with the L1 mutants showed impaired ability to extend axons on an L1 substrate but not on other control substrates. These results demonstrate the novel role of CKII in L1-mediated axon elongation and stress the importance of functional linkage between L1 phosphorylation and L1 trafficking in migrating growth cones.