Abstract: The Fuming trifoliate orange is rarely occurning wild Poncirus biotype growing in the primary mountain forests of Yunnan Province, southwestern China. It is different from the ordinary trifoliate orange ( P. trifoliata Raf. ) in morphology. It has 3-foliolate、evergreen leaves, ovary with 10 loculcs, petals 5—9, stamens 35—43, flowering 2—3 times in one year. Leaf extracts from this new biotype, ordinary trifoliate orange, Fortunella and Citrus were analyzed by polyacrylamide gel electrophoresis for isozymes of glutamate oxaloacetate transaminase (GOT), phosphoglucose mutase (PGM), phosphoglucose isomerase (PGI), peroxidase (PX), malie enzyme (ME), superoxide dismutase (SOD) and tetrazolium oxidase (TO). Alleles for 14 loci, namely Got-1, Got-2, Got-3, Pgm-1, Pgm-2, Pgi-1, Pgi-2, Px, Me-1, Me-2, Sod-1, Sod-2, To-1 and To-2 were determined. Two specific M alleles of Sod-1, a rare R allele of To-2 and null genes at the loci of To-1 and Got-3 suggested that the Fuming trifoliate orange could be accepted as a new species of the genus Ponctrus—P. polyandra S. Q. Ding et al. This species may be an evolutionary bridge and one of the "missing links" between Poncirus and Citrus.