This paper presents a technique that uses a genetic algorithm for automatic test-data generation. A genetic algorithm is a heuristic that mimics the evolution of natural species in searching for the optimal solution to a problem. In the test-data generation application, the solution sought by the genetic algorithm is test data that causes execution of a given statement, branch, path, or definition-use pair in the program under test. The test-data-generation technique was implemented in a tool called TGen. in which parallel processing was used to improve the performance of the search. To experiment with TGen, a random test-data generator, called Random, was also implemented. Both TGen and Random were used to experiment with the generation of test-data for statement and branch coverage of six programs.