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Phosphorus (P) deficiency is one of the common nutritional disorders in crop and horticultural plants. Various species and genotypes differ in tolerance to P deficiency. In this research, effects of low P supply were studied in two tomato (Solanum lycopersum L.) cultivars (Behta and Piazar) in hydroponics in order to introduce the most tolerant cultivar to P deficiency. In addition, we studied some involving mechanisms for differences in P deficiency tolerance between the two studied cultivars. Phosphorus deficiency caused reduction of shoot and root dry weight. Growth reduction due to P deficiency was greater in Behta than Piazar. Leaf pigments, chlorophyll fluorescence parameters and net assimilation rate were influenced by P deficiency more pronouncedly in Behta than Piazar. Low P supply caused accumulation of soluble carbohydrates, starch and free amino acids but resulted in reduction of soluble protein concentration. Concentration of P was decreased under low P conditions and relative allocation of P to the leaves, stems and roots were different between two cultivars, being higher for the leaves in Piazar and for the stems in Behta. Concentration of P in the old leaves decreased 58-67% while increased in the young leaves up to 220-350% which reflected the re-translocation of P from the former to the latter leaves. Although cultivars did not differ in the extent of re-translocation, it was higher under low phosphorus compared with adequate P supply. Our results suggested that, different tolerance to P deficiency in the two studied cultivars could be attributed to both differences in P acquisition and internal P use efficiency.