We present an experiment that gives insight into the origin of the dependence of the hole mobility (mu) on the dopant species in heavily doped p-type Si under low electrical field. The Hall carrier concentration and mobility were measured in Si coimplanted with B and Ga in the 0.1-2 x 10{20} cm{-3} concentration range. The strain induced by substitutional dopants, detected by high resolution x-ray diffraction, was varied by changing the relative B and Ga concentration. The effect of strain on mobility was disentangled and a linear relationship between 1/mu and the perpendicular strain was found.