A real-time quantitative Polymerase Chain Reaction (PCR) method has been developed for the quantitation of mule duck (Anas platyrhynchos x Cairina moschata) in binary duck/goose foie gras mixtures. The method combines the use of real-time PCR with duck-specific and endogenous control "duck + goose" primers to measure duck content and total foie gras content, respectively. Both PCR systems (duck-specific and duck + goose) were designed on the mitochondrial 12S ribosomal RNA gene (rRNA). The duck-specific system amplifies a 96 bp fragment from duck DNA, whereas the duck + goose system amplifies a 120 bp fragment from duck and goose DNA. The method measures PCR product accumulation through a FAM-labeled fluorogenic probe (TaqMan). The C(t) (threshold cycle) values obtained from the duck + goose system are used to normalize the ones obtained from the duck-specific system. Analysis of experimental duck/goose foie gras binary mixtures demonstrated the suitability of the assay for the detection and quantitation of duck in the range of 1-25%. This genetic marker can be very useful to avoid mislabeling or fraudulent species substitution of goose by duck in foie gras.