In the search for early-detectable selection criteria for growth at low temperature conditions in tomato, first the initiation and growth of individual leaves was analysed. Scanning electron microscopy revealed that the first four primordia had already developed during the germination period at 25°C. The primordium of the fifth leaf, however, was initiated after the transfer of seedlings to the experimental conditions. The increase in length of the first three leaves, and to a lesser extent of the fourth leaf, was considerably smaller in comparison with that of later formed leaves. Moreover, the morphology of the first three to four leaves was deviant, whereas the others showed the normal compound leaf architecture. All these results indicated that the fifth leaf was the earliest formed leaf with growth characteristics that might reflect the growth potential of the whole plant. Development of the fifth leaf was tested as a marker for whole plant growth. At three temperature, 18, 15 and 12°C, growth responses of the fifth leaf were similar to that of whole plants in four tomato genotypes: Line A, Line B, Premier and MXXIV-13. Significant differences in relative growth rate of dry weight of whole plants and fifth leaves (RGRW)and of leaf area of the fifth leaves (RGRLA between two fast growing and two slow growing genotypes were found. No genotype by temperature interaction for RGRW and RGRLA was found, indicating that the effect of temperature decrease was similar for the four genotypes. The structure of the mature fifth leaf of one fast and one slow growing genotype, Line A and MXXIV-13, was analysed. For both genotypes, leaves were small and thick at low temperature, 12°C. The total number of epidermis and palisade parenchyma cells per leaf was smaller but the size of the cells developed at 12°C was larger than at 18°C. Consequently, the slow growth at 12°C was due to a low rate of cell division. At both temperatures, the fifth leaf to MXXIV-13 was smaller compared to that of line A. Since the size of the cells were similar, the smaller leaf size was due to lower number of leaf cells. The results confirm the suitability of the growth, especially expressed as RGRLA , of the fifth leaf as a nondestructive market for vegetative development of tomato at low temperature. Growth differences between genotypes were mainly reflected by differences in cell number of leaves, which might be correlated with genetically determined differences in cell number of leaf primordia.