We report on a search for pair production of a fourth generation charge −1/3 quark ( b′) in pp¯ collisions at s=1.8 TeV by the DØ experiment at the Fermilab Tevatron using an integrated luminosity of 93 pb−1. Both b′ quarks are assumed to decay via flavor changing neutral currents (FCNC). The search uses the signatures γ+3 jets +μ-tag and 2γ+2 jets. We see no significant excess of events over the expected background. We place an upper limit on the production cross section times branching fraction that is well below theoretical expectations for a b′ decaying exclusively via FCNC for b′ masses up to mZ+mb.Received 26 November 1996DOI:https://doi.org/10.1103/PhysRevLett.78.3818©1997 American Physical Society