Mammalian Drp1 is a dynamin-like GTPase required for mitochondrial fission. Although it exists primarily as a cytosolic homo-tetramer in vivo, it can also self-assemble into higher order structures on the mitochondrial outer membrane, where it is required for proper mitochondrial division. Functional studies and sequence comparisons have revealed four different structural domains in Drp1, comprising N-terminal GTP-binding, middle, insert B, and C-terminal GTPase effector (GED) domains. Here we describe an intramolecular interaction within Drp1 between the GED and the N-terminal GTP-binding and middle domains. A point mutation (K679A) within the C-terminal GED domain inhibits this intramolecular association, without affecting the formation of Drp1 tetramers or the intermolecular associations among isolated C-terminal domains. Mutant Drp1 K679A exhibits impaired GTPase activity, and when overexpressed in mammalian cells it decreases mitochondrial division. Sedimentation experiments indicate that the K679A mutation either increases Drp1 complex formation or, more likely, decreases complex disassembly as compared with wild-type Drp1. Taken together, these data suggest that the C-terminal GED domain is important for stimulation of GTPase activity, formation and stability of higher order complexes, and efficient mitochondrial division.