Four electronically low-lying states of CH2 have been investigated systematically by using ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration self-consistent-field (TCSCF), complete active space self-consistent-field (CASSCF;), configuration interaction with single and double excitations (CISD), and CASSCF second-order configuration interaction (SOCI) levels of theory were employed with nine different basis sets. Special emphasis is placed on the second and third excited states. The third excited state ((c) over tilde (1)A(1)) is of particular theoretical interest because it is represented by the second root of TCSCF, CASSCF, TCSCF-CISD, and CASSCF-SOCI wave functions. Theoretical treatments of states not the lowest of their symmetry are traditionally treacherous. It is confirmed in this study that the four low-lying states of CH2 all have bent structures. For these four states total energies and physical properties including dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were determined and compared with available experimental values. At the CISD level with the largest basis set, the triple-zeta plus triple polarizations with two higher angular momentum and two diffuse functions basis set [TZ3P(2f,2d)+2diff], the dipole moments are determined to be 0.600 ((X) over tilde B-3(1)), 1.690 ((a) over tilde (1)A(1)), 0.669 ((b) over tilde B-1(1)), and 0.205 D ((c) over tilde (1)A(1)), respectively. With the most accurate method in this study, the CASSCF-SOCI level with the TZ3P(2f,2d)+2diff basis set, the energy separation (T-0 value) between the ground state ((X) over tilde B-3(1)) and the first excited state ((a) over tilde (1)A(1)) is predicted to be 9.025 kcal/mol (0.3914 eV, 3157 cm(-1)), which is in excellent agreement with an experimentally derived value of 8.998 kcal/mol (0.3902 eV, 3147 cm(-1)) by Jensen and Bunker (J. Chem. Phys. 1988, 89, 1327). The T-0 value for the second excited state ((b) over tilde B-1(1)) is determined to be 33.224 kcal/mol (1.4407 eV, 11 620 cm(-1)), which is in good agreement with an experimental estimate of 32.546 kcal/mol (1.4113 eV, 11 383 cm(-1)) by Jensen and Bunker above and by Alijah and Duxbury (Mol. Phys. 1990, 70, 605). For the third excited state ((c) over tilde (1)A(1)) the T-0 value is predicted to be 59.08 kcal/mol (2.562 eV, 20 660 cm(-1)) with estimated errors of +/-1.50 kcal/mol or +/-525 cm(-1). The equilibrium geometry of this (c) over tilde (1)A(1) state is determined to be r(e) = 1.064 Angstrom and theta(e) = 171.6 degrees at the TCSCF-CISD level with the largest basis set.