Abstract: An experiment was conducted in Harare (Zimbabwe) in 2004 to investigate the effect of substrate (wheat straw) quantity and shelf position on yield of oyster mushroom (Pleurotus sajor-caju) using plastic tray culture. The experiment was laid out as a 2-way factorial in a randomized block design with five substrate quantities (2, 4, 6, 8 and 10 kg of wheat straw per tray) and two shelf positions (0.5 and 1.2 m above ground). Mushroom yield increased with an increased substrate quantity of up to 6 kg and thereafter remained constant. Shelf position had no significant effect (p>0.05) on mushroom yield and there was no interaction (p>0.05) between substrate quantity and shelf position on yield. Biological efficiency decreased with an increase in substrate quantity per tray. It was concluded that 6 kg of substrate per tray (50x35 and 20 cm deep) would result in optimal yields. The height above the ground at which trays are placed in the growing room does not affect mushroom yields.