Mutation testing is a technique for unit-testing software that, although powerful, is computationally expensive, The principal expense of mutation is that many variants of the test program, called mutants, must be repeatedly executed.This article quantifies the expense of mutation in terms of the number of mutants that are created, then proposes and evaluates a technique that reduces the number of mutants by an order of magnitude.Selective mutation reduces.the cost of mutation testing by reducing the number of mutants, This article reports experimental results that compare selective mutation testing with standard, or nonselective, mutation testing, and results that quantify the savings achieved by selective mutation testing, The results support the hypothesis that selective mutation is almost as strong as nonselective mutation: in experimental trials selective mutation provides almost the same coverage as nonselective mutation.with a four-fold or more reduction in the number of mutants.