A tetrapolar mating system was found in the 11 isolates of P. ostreatus used in this study. Seventeen different A and 20 different B factors were detected. Recombination within the A series averaged 0.5% and 4.5% at the B series at 25-27[degree]C. In growth studies, interstock (hybrid) dikaryons produced an average yield of 716.1 g of oyster mushrooms while the intrastock (sibling) dikaryons averaged 540.2 g when grown on aspen sawdust supplemented with whole grain oats.