Electronic states of the CH3Se and its cation CH3Se+ have been studied using the complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) methods in conjunction with the ANO-RCC(TZP) basis set. To investigate the Jahn-Teller effect on the CH3Se radical, C-s symmetry was used for CH3Se in calculations. The results show that the Jahn-Teller effect is very small (69 cm(-1)) and the 1(2)A '' state is slightly more stable than the 1(2)A '' state (8 cm(-1)). The CH3Se has been found to have a 1(2)A' ground state with a C-Se bond distance of 1.975 angstrom. The computed C-Se stretching v(6)(a') frequency is 554.1 cm(-1), which is in good agreement with the experimental values of 600 +/- 60 cm(-1). The calculations for CH3Se at 3.621 and 5.307 eV are attributed to 1(2)A' -> 2(2)A'(1(2)A(1)) and 1(2)A' -> 2(2)A '', respectively. The vertical and adiabatic ionization energies were obtained to compare with the PES data.