It has been demonstrated that synapses lacking functional synaptotagmin I (Syt I) have a decreased rate of synaptic vesicle endocytosis. Beyond this, the function of Syt I during endocytosis remains undefined. Here, we demonstrate that a decreased rate of endocytosis in syt^null mutants correlates with a stimulus-dependent perturbation of membrane internalization, assayed ultrastructurally. We then separate the mechanisms that control endocytic rate and vesicle size by mapping these processes to discrete residues in the Syt I C₂B domain. Mutation of a poly-lysine motif alters vesicle size but not endocytic rate, whereas the mutation of calcium-coordinating aspartate residues (syt-D3,4N) alters endocytic rate but not vesicle size. Finally, slowed endocytic rate in the syt-D3,4N animals, but not syt^null animals, can be rescued by elevating extracellular calcium concentration, supporting the conclusion that calcium coordination within the C₂B domain contributes to the control of endocytic rate.