  Let $P$ and $T$ be disjoint sets of prime numbers with $T$ finite. A simple
formula is given for the natural density of the set of square-free numbers
which are divisible by all of the primes in $T$ and by none of the primes in
$P$. If $P$ is the set of primes congruent to $r$ modulo $m$ (where $m$ and $r$
are relatively prime numbers), then this natural density is shown to be $0$.
