Abstract. In this paper we study and characterize those Diophantine inequalities ax mod b ≤ x whose set of solutions is a symmetric numerical semigroup.Given two integers a and b with b = 0 we write a mod b to denote the remainder of the division of a by b. Following the notation used in [8], a modular Diophantine inequality is an expression of the form ax mod b ≤ x. The set S(a, b) of integer solutions of this inequality is a numerical semigroup, that is, it is a subset of N (the set of nonnegative integers) closed under addition, 0 ∈ S(a, b) and such that N\S(a, b) has finitely many elements. We say that a numerical semigroup is modular if it is the set of solutions to a modular Diophantine inequality. As shown in [8], not every numerical semigroup is of this form.If S is a numerical semigroup, then the greatest integer not in S is the Frobenius number of S, denoted by g(S). The numerical semigroup S is symmetric (see [1]) if x ∈ Z \ S implies g(S) − x ∈ S (Z is the set of integers). This kind of semigroup has been widely studied and characterized in the literature (see, for instance, [2,4,6]). We will say that the inequality ax mod b ≤ x is symmetric if S(a, b) is a symmetric numerical semigroup.It is well known (see, for instance, [7]) that every numerical semigroup S is finitely generated and thus there exist positive integers n 1 , . . . , n p such thatIf no proper subset of {n 1 , . . . , n p } generates S, then we say that this set is a minimal system of generators of S. Minimal systems of generators always exist and are unique (see [7]); the cardinality of a minimal system of generators of S is known as the embedding dimension of S, denoted here by e(S).Clearly, the inequality ax mod b ≤ x has the same integer solutions as the inequality (a mod b)x mod b ≤ x. Thus we may assume (and in fact we will) that a, b ∈ N and a < b. Note that S(0, b) = N is trivially symmetric.Throughout this paper (and unless otherwise stated) we will assume that a and b are positive integers, and that d = gcd{a, b} and d = gcd{a − 1, b} (gcd stands for greatest common divisor). In Proposition 4, we will show that S