The tomato cv. ‘Santa Clara’ is widely cultivated among tomato producers in most of the South-east of Brazil. Recently, some plants of this cultivar were identified with morphological alterations in both vegetative and reproductive organs. These plants showed firm (firme) ripe fruits, slow and delayed ripening. They also had yellow leaves associated with precocious senescence and flowers with pale stigmas. The objective of this work was to determine the genetic model of inheritance for this mutation and to evaluate its effects on shelf life and loss of firmness in mature fruits, as well as analyse the occurrence of genetic relationships between this putative mutant and other pleiotropic mutants. Mutated plants were crossed with the non-mutant cv. ‘Santa Clara’ and some previously described pleiotropic mutants. Seeds of F₁ and F₂ generations and backcrosses were obtained for the segregation analysis. Morphological characteristics modified by this mutation are governed by a recessive gene with pleiotropic effects. In addition, the test of allelism showed a lack of genetic complementation between the ‘firme’ mutant and lutescent-2 mapped on chromosome 10. Fruits of the ‘firme’ mutant had a slower rate of softening compared with the cv. ‘Santa Clara’ and its hybrids. The fruit shelf life of the mutant ‘firme’ was significantly superior to the other genotypes. No maternal effect was detected in either qualitative or quantitative characteristics. Based on the data, the mutation ‘firme’ in the cv. ‘Santa Clara’ is located in the region containing the l-2 locus, which promoted alterations in ripening and post-harvest physiology of fruits. The mutation ‘firme’ may represent a new allele of the gene lutescent-2 or a gene linked to physiological events of fruit ripening.