The equilibrium structure and potential energy surface of calcium monohydroxide in its ground doublet state, X(2)Sigma(+) CaOH, have been determined from large-scale ab initio calculations using the spin-restricted coupled-cluster method, RCCSD(T), with basis sets of quadruple- and quintuple-zeta quality. The vibrational-rotational energy levels of the CaOH and CaOD isotopomers were calculated using the variational method. The spectroscopic constants determined are found to be in remarkably good agreement with experimental data.