We employ pseudopotential plane-wave calculations to study the interstitial B in Si in different ionic configurations and charge states. For all charge states the ground state is a B-Si pair in which the B atom is close to a substitutional site and the Si atom in a nearby tetrahedral position. The defect has negative-U property and exhibits a symmetry-lowering distortion. We also report several metastable configurations which are close in formation energy. The relation of the defects to B diffusion is discussed.