Tomato plants were grown in a climate chamber in water culture at standard nutrient solution concentration with electrical conductivity of 2 mS-cm-1. At the start of the development of the fourth cluster the EC was increased to 6, 9 or 12 mS-cm-1, resulting in a water potential of the nutrient solution of -0.2, -0.3 or -0.4 MPa, respectively. We measured the growth in volume of all fruits with an electronic calliper and at specific periods individual fruit growth using displacement transducers. Plant water potential was measured with a pressure chamber. Fruit water status was determined by measuring the osmotic potential of the pericarp. Above a threshold value the total fruit growth rate was linearly related to EC, with a sensitivity of about 10ÈmS-cm-1)-1. The effect of EC on fruit growth was not only dependent on the level of EC, but also on the length of exposure to high EC during fruit development. Plant water potential changed concomitantly with EC. Fruit osmotic potential lagged behind and was also dependent on the length of exposure to high EC. We consider the water potential gradient between plant and fruit to be the driving force for import of water into the fruit and present a fruit growth simulation on the basis of the relationships between root environment water potential and plant-to-fruit water potential gradient.