Ventilation is one of the most important factors in the design of greenhouses since it directly affects the average air temperature, humidity and CO2 concentration within the enclosed space. For any greenhouse, ventilation performance is a major factor in production, influencing the yield and quality of the products. In evaluating the effectiveness of ventilation, air exchange rate is taken as a criterion. An air exchange rate of 60 h(-1) is required to maintain good ventilation. This study was conducted to determine the effects of ventilation opening types on air exchange rate in the natural ventilation of greenhouses in the Western Mediterranean Region of Turkey. Greenhouse enterprises totaling 188 were surveyed in the study area in 2008. There were seven different types of greenhouses based on cover material, occurrence as single or multiple structures and type of ventilation. One from each group was subsequently selected to examine the air exchange rates using long-term climatic data as a function of position of ventilation opening (roof, sidewall and both roof and sidewall). The effects of different ventilation opening types on air exchange rate were found to be statistically significant (p<0.05). The air exchange rates in greenhouses ventilated with both roof and sidewall were above 60 h(-1) during the experimental period, and these rates were significantly higher than those for the other two ventilation types.