An assembly consists of two or more mating parts. The quality of the assembly is mainly based on the quality of mating parts. The mating parts may be manufactured using different machines and processes with different standard deviations. Therefore, the dimensional distributions of the mating parts are not similar. This results in clearance between the mating parts. To obtain high precision assemblies, clearance variation has to be reduced. Selective assembly helps to reduce this clearance variation. In this paper, appropriate selective group combination for assembling the mating parts is obtained using an ant colony optimization (ACO). The combination obtained has resulted in an appreciable reduction in clearance variations. © 2011 Springer-Verlag.