Pectinmethylesterase, an enzyme involved in cell wall softening of papaya fruit was isolated. The structure of this cDNA and its expression during development and ripening of the fruit was analysed. Northern Blotting and was used to determine the expression of pectinmethylesterase genes along fruit development and ripening. PME is differentially expressed in the inner and outer mesocarp. The levels of PME activity increase gradually with maturation until day 7 of ripening. The pectinmethylesterase activity increases differentially from the outer mesocarp to the inner mesocarp along ripening. These values are similar for 7 days ripened fruits which corresponds to 70% ripening. After that ripening stage there are no significant differences between PME in inner and outer mesocarp and the PME activity is reduced of about 10%. The phylogram generated using an alignment of the deduced amino acid sequences of PME and of 10 PMEs homologues from other plant species revealed that pectinmethyl esterase from papaya fruits presents higher similarity with tomato PME sequences than with the other PMEs sequences available. The amount of total RNA in the mature ripe fruit duplicated the amount of total RNA in the green fruit. All the cDNAs were expressed at similar levels at the inner and outer mesocarp tissues during the different stages of fruit ripening. However, its expression was highest for ripening stages 1, 3, 5 and 7 decreasing thereafter to lower levels of expression. These results show that increase in mRNA translation parallels the increase in PME activity until day 7 of ripening.