The (0,0) band of the B (2)Sigma(+)-X (2)Sigma(+) transition of LaS near 726 nm has been studied at high resolution using laser vaporization/reaction supersonic free jet expansion and laser induced fluorescence spectroscopy. Spectra taken at a resolution of about 50 MHz show resolved hyperfine structure which is caused principally by the unpaired 6ssigma electron in the ground state interacting with large magnetic moment of the La-139(57) nucleus with nuclear spin I=7/2. The X (2)Sigma(+) state of LaS is in good Hund's case (b(betas)) coupling, however, the upper state, B (2)Sigma(+), is close to case (b(betaJ)) coupling. A total of 1850 rotational-hyperfine transition lines have been assigned and measured. Accurate rotational, spin and hyperfine parameters for the B (2)Sigma(+) and X (2)Sigma(+) states have been obtained, which give information about the electron distribution in the molecule.