ABSTRACT In this research the abscission and fruit growth process were studied on sweet orange fruit in 20-year-old trees of Citrus sinensis (L.) Osbeck. cv Salustiana with a high flowering level. The influence of 2,4-D applications in full bloom on this process was also investigated. The study was carried out during two consecutive years. The abscission was determined on the whole tree and on the leafy inflorescences. The growing variables of the fruit were characterized (diameter, fresh and dry weight), as well as the sugar content and the invertase enzymatic activity. The invertase activity was determined according to the procedure described by Somogy-Nelson and the sugar content was determined by High Performance Liquid Chromatographic. Most of the initially formed flowers fall at different stages of development. The abscission extends till the middle of June (day 74). Abscission is presented in absolute and relative terms. Both parameters reveal two main peaks of abscission. The abscission trend in leafy inflorescences is initially determined by a reduction in the number of fruits for inflorescences, followed by a reduction in the number of inflorescences with fruit. The fruit set is greater in multifloral leafy inflorescences than in unifloral leafy inflorescences. The increase in the growing variables of the fruit (diameter, fresh and dry weight) is slow till day 33 when the peel shows a higher contribution to the fruit cross-sectional area. From this day in advance the growth rate of the fruit increases; the peel shows a progressive growth between days 35 and 68 when it is stabilized and the locules show a slow growth till day 45, after this day, the growth of the locules contributes to a higher extent to the cross sectional area (from day 65 to day 98). Initially the zone that shows the highest sugar accumulation is the peel. By day 76, once the expansion of the locules starts, the highest sugar accumulation was observed in the vesicles. During anthesis the invertase acid activity is greater, this activity decreases as the fruit development progresses. The greatest invertase alkaline activity occur when the vesicles cellular expansion starts and therefore the sugar accumulation in this zone. The response of the fruit to 2,4 D application besides being dependent from the level of auxin concentration, it is also determined by the amount of applied solution. The year 2003, 2,4 D was sprayed with a concentration of 15 mg/l, 7,7 l per tree. The addition of the auxin did not alter the average weight of the fruits, however it caused an increase in the fruit set leading to an increase in the yield at harvest. The year 2004, the irrigation of the auxin (20 mg /l, 3.6 l per tree) increased the growth rate of the fruits and the fruits size at maturity, however reduced the number of fruits which kept constant the yield at harvest. The year 2004, the differences between the diameter of the control fruits and the fruits treated with 2,4-D are observed during the early fruitlet development. These differences increase with time following a linear relationship. For all the studied variables the diary increase level reaches the maximum by day 53, when the cell expansion of the vesicles starts. 2,4-D application enhances the sugar level in the fruit and affect the activity of the invertases isoforms. The increase in the acid invertase isoform activity suggests a direct effect of the 2,4-D on the fruit sink strength, which could be related with the greater sugar consume during the early fruit development. Concentrations of sucrose and reducing sugars in the vesicles as observed by 2,4-D application occur at the starts of the vacuolization of the vesicles cells. It could be demonstrated a participation of the alcalin invertases in the processes of sugar accumulation in the vesicles. In the year 2002 the behaviour in vitro of the fruits tissues under the addition of AIA to the medium was evaluated following experimental methods previously described by Guardiola et al., 1993. The growth pattern in vitro of the fruit, besides being determined by the hormonal treatment, it is also determined by the stage of development of the fruit from which the inoculums is extracted and by the inoculums size. It is observed a loss in the response capacity of the fruit tissues to the AIA as the stage of fruit development is more advanced.