Greenhouse ventilation depends on parameters such as internal and external climates and the design of the greenhouse and its vents. These have been intensively studied and results incorporated into various general ventilation models in the past. However, ventilation also greatly depends on the characteristics of the crop occupying the greenhouse space, especially with tall crops such as tomatoes. The effect of the crop is particularly great for greenhouse tunnels, whose side openings may be partly obstructed by the crop rows. In the past very few studies have been made of this crucial question. To address the issue, the relationship between ventilation of a tunnel type greenhouse and the characteristics of the tomato crop growing in it was systematically studied. The greenhouse tunnel's natural ventilation was studied experimentally, using the tracer gas method, for three main stages of the tomato crop: (i) with small plants; (ii) with a young tomato crop 1 m high; (iii) with a mature tomato crop 2.13 m high. The dependence of ventilation efficiency in the greenhouse tunnel on crop height, leaf area index and wind direction was assessed and numerical relations deduced