In this paper, the automorphism group of a generalized extraspecial p-group G is determined, where p is a prime number. Assume that |G| = p 2n+m and |ζG| = p m , where n ⩾ 1 and m ⩾ 2. (1)
When p is odd, let Aut G′ G = {α ∈ AutG | α acts trivially on G′}. Then Aut G′ G ⊲ AutG and AutG/Aut G′ G ≌ ℤ p−1. Furthermore
(i)
If G is of exponent p m , then Aut G′G/InnG ⊲ Sp(2n, p) × ℤ p m−1.
(ii)
If G is of exponent p m+1, then Aut G′ G/InnG ⊲ (K ⋊ Sp(2n − 2, p)) × ℤ p m−1, where K is an extraspecial p-group of order p2n−1.
In particular, Aut G′ G/InnG ⊲ ℤ p × ℤ p m−1 when n = 1. (2)
When p = 2, then (i)
If G is of exponent 2 m , then AutG ⊲ Sp(2n, 2) × ℤ2 × ℤ2 m−2.
In particular, when n = 1, |AutG| = 3 · 2 m+2. None of the Sylow subgroups of AutG is normal, and each of the Sylow 2-subgroups of AutG is isomorphic to H ⋊ K, where H = ℤ2 × ℤ2 × ℤ2 × ℤ2 m−2, K = ℤ2.
(ii)
If G is of exponent 2 m+1, then AutG ⊲ (I ⋊ Sp(2n − 2, 2)) × ℤ2 × ℤ2 m−2, where I is an elementary abelian 2-group of order 22n−1.
In particular, when n = 1, |AutG| = 2 m+2 and AutG ⊲ H ⋊ K, where H = ℤ2 × ℤ2 × ℤ2 m−1, K = ℤ2.