Katanin, a member of the AAA adenosine triphosphatase (ATPase) superfamily, uses nucleotide hydrolysis energy to sever and disassemble microtubules. Many AAA enzymes disassemble stable protein-protein complexes, but their mechanisms are not well understood. A fluorescence resonance energy transfer assay demonstrated that the p60 subunit of katanin oligomerized in an adenosine triphosphate (ATP)- and microtubule-dependent manner. Oligomerization increased the affinity of katanin for microtubules and stimulated its ATPase activity. After hydrolysis of ATP, microtubule-bound katanin oligomers disassembled microtubules and then dissociated into free katanin monomers. Coupling a nucleotide-dependent oligomerization cycle to the disassembly of a target protein complex may be a general feature of ATP-hydrolyzing AAA domains.