Acyclicity constraints are prevalent in knowledge representation and, in particular, applications where acyclic data structures such as DAGs and trees play a role. Recently, such constraints have been considered in the satisfiability modulo theories (SMT) framework, and in this paper we carry out an analogous extension to the answer set programming (ASP) paradigm. The resulting formalism, ASP modulo acyclicity, offers a rich set of primitives to express constraints related with recursive structures. The implementation, obtained as an extension to the state-of-the-art answer set solver clasp, provides a unique combination of traditional unfounded set checking with acyclicity propagation.