The ESCRT machinery, including ESCRT-III, localizes to the midbody and participates in the membrane abscission step of cytokinesis. The ESCRT-III protein CHMP1B is required for recruitment of the MIT domain-containing protein spastin, a microtubule severing enzyme, to the midbody. The 2.5 Å structure of the C-terminal tail of CHMP1B with the MIT domain of spastin reveals a specific, high-affinity complex involving a non-canonical binding site between the first and third helices of the MIT domain. The structural interface is twice as large as that of the MIT domain of VPS4-CHMP complex, consistent with the high affinity of the interaction. A series of unique hydrogen bonding interactions and close packing of small side-chains discriminate against the other ten human ESCRT-III subunits. Point mutants in the CHMP1B binding site of spastin block recruitment of spastin to the midbody.