  In this paper we investigate weighted norm inequalities for the commutator of
a fractional integral operator and multiplication by a function. In particular,
we show that, for $\mu,\lambda\in A_{p,q}$ and $\alpha/n+1/q=1/p$, the norm $\|
[b,I_\alpha]:L^p(\mu^p)\to L^q(\lambda^q) \|$ is equivalent to the norm of $b$
in the weighted BMO space $BMO(\nu)$, where $\nu=\mu\lambda^{-1}$. This work
extends some of the results on this topic existing in the literature, and
continues a line of investigation which was initiated by Bloom in 1985 and was
recently developed further by the first author, Lacey, and Wick.
