SUMMARYWhen mixed into the casing or compost layers of a mushroom bed in the absence of pests, bendiocarb decreased yield and number of mushrooms according to concentration. The most severe effects were on mushroom number at the two highest rates used (100 and 1000 μg/g), and there were large increases in mushroom size. Effects of bendiocarb incorporation in the compost diminished with time, and there was partial compensation in yield and numbers at the fourth flush. The action of bendiocarb persisted when it was mixed into the casing.Diflubenzuron showed some opposite effects at lower concentrations. When either mixed into, or drenched onto the casing at the commercial rate (30 μg/g), yield and size were both increased and the timing of the flushes was unaffected. At the two higher concentrations (180 and 1080 μg/g), reductions in yield and number and an increase in mushroom size were shown. However, these effects became more severe with time, especially those on mushroom number, possibly due to the accumulation of a toxic breakdown produ