Multireference singles and doubles configuration interaction calculations which include relativistic effective core potentials have been performed on GaAs. Potential energy curves of 40 Lambda-S states are computed. Spectroscopic constants (T-e, r(e), and omega(e)) of 17 bound Lambda-S states have been estimated and compared with the available observed and other calculated values. The lowest Lambda-S state is ...pi(2) X(3)Sigma(-). All 22 Lambda-S states which dissociate into Ga(P-2(u)) + As(S-4(u)) and Ga(P-2(u)) + As(D-2(u)) limits are taken into consideration for the spin-orbit CI calculations. Potential energy curves of 51 Omega-states are computed. The X(3)Sigma(0+)(-) component is found to be the ground state of GaAs in contrary to the experimental findings which suggest the other component X(3)Sigma(1)(-) as the ground state. The splitting between the two components of X(3)Sigma(-) is calculated to be 76 cm(-1) as compared with the experimental value of 43 cm(-1). The excited A(3)Pi(0+) state, which does not predissociate has r(e) = 2.757 Angstrom, T-e = 22 178 cm(-1), and omega(e) = 125 cm(-1). T-e and omega(e) values are somewhat smaller, while r(e) is larger than the observed data. Transition probabilities of several dipole allowed transitions are calculated. Three transitions such as 1(1)Sigma(+)-4(1)Sigma(+), 2(1)Sigma(+)-4(1)Sigma(+), and X(3)Sigma(-)-A(3)Pi are found to be very strong. The radiative lifetimes of many excited states are also estimated. The A(3)Pi(0+) state at the vibrational level upsilon＇ = 0 has the calculated radiative lifetime of 1050 ns which is somewhat larger than the observed range.