Satsuma (Citrus unshiu [Mak] Marc.) and Clementine (Citrus reticulata [Hort.] Ex. Tanaka, cv Oroval) are two species of seedless mandarins differing in their tendency to develop parthenocarpic fruits. Satsuma is a male-sterile cultivar that shows a high degree of natural parthenocarpy and a high fruit set. Seedless Clementine varieties are self-incompatible, and in the absence of cross-pollination show a very low ability to set fruit. The gibberellins (GAs) GA53, putative 17-OH-GA(53), GA(44), GA(17), GA(19), GA(20), GA(29), GA(1), 3-epi-GA(1), GA(8), GA(24), GA(9), and GA(4) have been identified from developing fruits of both species by full-scan combined gas chromatography-mass spectrometry. Using selected ion monitoring with [(2)H(2)]- and [(13)C]-labeled internal standards, the levels of GA(53), GA(44), GA(19), GA(20), GA(1), GA(8), GA(4), and GA(9) were determined in developing ovaries at anthesis and 7 days before and after anthesis, from both species. Except for GA8, levels of the 13-hydroxy-GAs were higher in Satsuma than in Clementine, and these differences were more prominent for developing young fruits. At petal fall, Satsuma had, on a nanograms per gram dry weight basis, higher levels of GA(53) (10.4x), GA(44) (13.9x), GA(19) (3.0x), GA(20) (11.2x), and GA(1) (2.0x). By contrast, levels of GA(8) were always higher in Clementine, whereas levels of GA(4) did not differ greatly. Levels of GA(9) were very low in both species. At petal fall, fruitlets of Satsuma and Clementine contained 65 and 13 picograms of GA(1), respectively. At this time, the application of 25 micrograms of paclobutrazol to fruits increased fruit abscission in both varieties. This effect was reversed by the simultaneous applications of 1 microgram of GA(3). GA(3) alone improved the set in Clementine (13x), but had little influence on Satsuma. Thus, seedless fruits of the self-incompatible Clementine mandarin may not have adequate GA levels for fruit set. Collectively, these results suggest that endogenous GA content in developing ovaries is the limiting factor controlling the parthenocarpic development of the fruits.