n a greenhouse without carbon fertilization, the CO2 absorbed in the process of photosynthesis must ultimately come from the external ambient through the ventilation openings. The ventilation of the greenhouse implies a trade-off between ensuring inflow of carbon dioxide and maintaining an adequate temperature within the house, particularly during sunny but chilly days. Crop production is known to increase both with carbon dioxide concentration and with [average] temperature. Therefore, the management of ventilation in such conditions is looking for ¿the lesser of two evils¿. After recalling the conclusion of a previous paper that carbon fertilization up to at least external concentration is the surest and cheapest way to increase productivity in such conditions, we deal with the question of optimal fertilisation in presence of natural ventilation. Allowing for a higher than external concentration obviously reduces the efficiency of the supply, but it does not necessarily reduce profit. By applying some economics to a simple assimilation model, we show that in many conditions¿particularly with relatively high radiation¿ maintaining higher than external concentrations does make economic sense, certainly up to ventilation rates of 10 per hour. We conclude that the optimal management of carbon fertilisation should aim at concentrations well above 1000 vpm in the absence of ventilation, and gradually decrease to maintaining the external value at ventilation rates well in excess of 10 per hour. Market conditions (value of produce v price of CO2) should determine the trend between these two extremes, that is, how fast or gradually should a grower limit fertilisation to only maintaining the external concentration.