  We study the prime values of Ramanujan's tau function $\tau(n)$. Lehmer found
that $n=251^2=63001$ is the smallest $n$ such that $\tau(n)$ is prime:
$$\tau(251^2)=-80561663527802406257321747.$$ We prove that in most arithmetic
progressions (mod 23), the prime values $\tau$ belonging to the progression
form a thin set. As a consequence, there exists a set of primes of Dirichlet
density $\frac{9}{11}$ which are not values of $\tau$.
