Activity and expression of polygalacturonase (PG), a hydrolytic enzyme involved in ultrastructural changes in the pericarp of sweet pepper (Capsicum annaum), were investigated at different ripening stages of the pepper cultivars Mandi and Talanduo. Molecular cloning of CaPG was carried out by constructing a cDNA library from three stages of fruit ripening. Morphological determination, PG assay, RT-PCR, and ultrastructural studies were used to quantify changes in CaPG gene expression in the pericarp from green, color change and fully ripened stages. We found that CaPG gene expression, PG activity and striking changes in the structure of the cell wall occurred with the transition of ripening stages. CaPG gene expression was high (obvious PCR products) in mature and ripened stages of both cultivars; however, the CaPG gene was not expressed in preclimacteric fruits or vegetative tissues. We conclude that developmental regulation of CaPG gene expression is instrumental for sweet pepper fruit ripening; its expression during development leads to dissolution of middle lamella and eventually disruption of the fully ripened cell wall.