  In this article, we extend Moon's classic formula for counting spanning trees
in complete graphs containing a fixed spanning forest to complete bipartite
graphs. Let $(X,Y)$ be the bipartition of the complete bipartite graph
$K_{m,n}$ with $|X|=m$ and $|Y|=n$. We prove that for any given spanning forest
$F$ of $K_{m,n}$ with components $T_1,T_2,\cdots,T_k$, the number of spanning
trees in $K_{m,n}$ which contain all edges in $F$ is equal to $$ \frac
1{mn}\prod_{i=1}^k (m_in+n_im) \left
(1-\sum_{i=1}^{k}\frac{m_in_i}{m_in+n_im}\right ), $$ where $m_i=|V(T_i)\cap
X|$ and $n_i=|V(T_i)\cap Y|$ for $i=1,2,\cdots,k$.
