The molecular parameters of chlorofulminate, ClCNO, were determined in large-scale ab initio calculations using the coupled-cluster method, CCSD(T), and basis sets of double-through quintuple-zeta quality. With the largest basis set employed, the equilibrium structure of the molecule was found to be bent, with the parameters r(ClC) = 1.6490 Angstrom, r(CN) = 1.1749 Angstrom, r(NO) = 1.2039 Angstrom, angle(ClCN) = 152.22 degrees, and angle(NCO) = 171.50 degrees. The potential energy function for the large-amplitude ClCN bending motion (the nu(5) mode) was determined to be strongly anharmonic, with a barrier to linearity of the ClCNO chain of 156 cm(-1). The rotation-bending energy levels were then calculated using a semirigid-bender Hamiltonian. The effective rotational constants determined for various nu(5) states were found to be in excellent agreement with the recent experimental data.