  As a consequence of the classification of finite simple groups, the
classification of permutation groups of prime degree is complete, apart from
the question of when the natural degree $(q^n-1)/(q-1)$ of ${\rm PSL}_n(q)$ is
prime. We present heuristic arguments and computational evidence based on the
Bateman-Horn Conjecture to support a conjecture that for each prime $n\ge 3$
there are infinitely many primes of this form, even if one restricts to prime
values of $q$. Similar arguments and results apply to the parameters of the
simple groups ${\rm PSL}_n(q)$, ${\rm PSU}_n(q)$ and ${\rm PSp}_{2n}(q)$ which
arise in the work of Dixon and Zalesskii on linear groups of prime degree.
