A comparison was conducted between ON-OFF and variable frequency drive (VFD) systems to control greenhouse ventilation fans. The study aimed to determine the effect of each system on the energy consumption and resulting greenhouse microclimate. The experiments were conducted in a commercial size greenhouse in which pepper was grown. To check the performance of the fan that was controlled by a VFD system, it was installed in a test facility and operated under several rotation speeds. At each speed of rotation, the static pressure on the fan was changed and parameters, such as electricity consumption and air flow rate, were measured. Reducing the fan speed with the VFD system resulted in reductions in the air flow rate through the greenhouse and energy consumption, the latter being much more significant. The study showed that VFD control can reduce electricity consumption compared with ON-OFF operation by an amount that depends on the weather. In the present study, the average energy consumption with the VFD control system over a period of one month, was about 0.64 of that with an ON-OFF system. The average greenhouse daily air temperatures and humidity ratios obtained with each control system between 0700 and 1800 were nearly equal during that month. The results obtained in the greenhouse further show that the VFD system has a greater potential than the ON-OFF to reduce the range of amplitude variations in the air temperature and humidity ratio within the greenhouse. (C) 2003 Elsevier Ltd. All rights reserved.