  The minimal standard model of quarks and leptons is extended with a set of
vectorlike fermions to allow baryon number $B$ to become a gauged $U(1)_B$
symmetry. The $B$ assignments of the new particles are determined by
renormalizable interactions with the known quarks through a color triplet
scalar diquark. The spontaneous breaking of $U(1)_B$ by a scalar with $B=3$
results in a conserved residual global $B$ symmetry. A singlet neutral scalar
with $B=2$ is a possible long-lived dark-matter candidate.
