Let f (x) = x n + ax 2 + bx + c ∈ Z[x] be an irreducible polynomial with b 2 = 4ac and let K = Q(θ) be an algebraic number field defined by a complex root θ of f (x). Let Z K deonote the ring of algebraic integers of K. The aim of this paper is to provide the necessary and sufficient conditions involving only a, c and n for a given prime p to divide the index of the subgroup Z[θ] in Z K . As a consequence, we provide families of monogenic algebraic number fields. Further, when2010 Mathematics Subject Classification. 11R04; 11R29. Key words and phrases. Ring of algebraic integers; Integral basis and discriminant; Monogenic number fields.By virtue of Corollary 1.2, Z K = Z[θ] if and only if for each prime p dividing D f , p 2 does not divide c when p | c and if p ∤ c, then p 2 ∤ D f . For example, if we take n = 5 and c = ±1 is a squarefree integer, then {1, θ, θ 2 , θ 3 , θ 4 } is an integral basis of K if and only if (3125 − 108c) is squarefree. It can be verified that (3125 − 108c) is squarefree when c = −3, 5, 13, 17, 21.