Is the b quark mixed with a vectorlike quark?

We examine the possibility of an exotic vectorlike charge (-(1/3)) quark h mixing with the d-type quarks. The currently accepted Cabibbo-Kobayashi-Maskawa matrix elements, though consistent with unitarity, allow substantial mixing of such an exotic quark with the b quark. Concentrating on h-b mixing, we find several distinctive signatures---in top-quark production and decay, Z→ bar bb, and flavor-violating rare Z decays---which may be experimentally testable in the near future