  We derive an asymptotic formula which counts the number of abelian extensions
of prime degrees over rational function fields. Specifically, let $\ell$ be a
rational prime and $K$ a rational function field $\Bbb F_q(t)$ with $\ell \nmid
q$. Let $\textup{Disc}_f\left(F/K\right)$ denote the finite discriminant of $F$
over $K$. Denote the number of abelian $\ell$-extensions $F/K$ with
$\textup{deg}\left(\textup{Disc}_f(F/K)\right) = (\ell-1)\alpha n$ by
$a_{\ell}(n)$, where $\alpha=\alpha(q, \ell)$ is the order of $q$ in the
multiplicative group $\left(\Bbb Z/\ell \Bbb Z\right)^\times$. We give a
explicit asymptotic formula for $a_\ell(n)$. In the case of cubic extensions
with $q\equiv 2 \pmod 3$, our formula gives an exact analogue of Cohn's
classical formula.
