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Genes encoding chalcone synthase (CHS), anthocyanidin synthase (ANS), and UDP-glucose-fl avonoid 3-O- glucosyltransferase (UFGT), some of the enzymes of anthocyanin biosynthetic pathway, were assayed in two different experiments using quantitative real-time reverse transcriptase (RT)-PCR, in order to test their transcription levels in the fl esh of blood and common orange (Citrus sinensis (L.) Osbeck) fruit, and to investigate their role in anthocyanin accumulation in the same tissue. The fi rst experiment compared a blood orange and a common orange cultivar during seven different fruit maturation stages. This was followed by the test of 11 different genotypes at the end of the winter season. Data collected from the fi rst experiment, over the blood orange cultivar, were statistically analyzed using the Pearson correlation coeffi cient. Results show that CHS, ANS, and UFGT mRNA transcripts are up- and co-regulated in the blood orange cultivar, whereas they are down-regulated in the common orange cultivar. There is evidence of correspondence between the target genes expression level and the content of the pigment assessed. The second test confi rms this correlation and shows that enzyme synthesis levels and pigment accumulation, in plants grown under the same environmental conditions, are dependent on the differences occurring among the genotypes tested. These results suggest that the absence of pigment in the common orange cultivars may be caused by the lack of induction on the structural genes expression. This is the fi rst report on the characterization of the relationships between biosynthetic genes expression and fruit fl esh anthocyanin content in blood oranges. The blood orange is the typical product of Sicilian citriculture and its red to purple pigmented rind and fl esh makes the fruit more attractive. Red color in blood oranges is due to the pres- ence of anthocyanins, the largest class of pigments among the fl avonoids. They are secondary metabolites, widely spread in higher plants and produced in different organs, in response to a variety of developmental, environmental, and genetic cues (Mol et al., 1996). They serve various reproductive functions (e.g., in fl ower petals to attract pollinators, and in seeds and fruit to aid in seed dispersal (Holton and Cornish, 1995)) and they protect plant tissues against pathogens and damage from environmental conditions (e.g., ultraviolet (UV) irradiation, cold temperature (Dixon and Paiva, 1995) and water stress (Chalker-Scott, 1999)). This class of pigmented molecules also possesses some important therapeutic properties for human health (Saija, 1994), function- ing as antioxidants, anti-infl ammatories, in epithelium repair and protection, and in modulation of capillary permeability. In conferring these properties to blood orange fruit, anthocyanins contribute to their commercial popularity and value, therefore research has been focusing on understanding the biosynthetic pathway for anthocyanins in oranges (Lo Piero et al., 2005