In the facultative Crassulacean Acid Metabolism (CAM) plant Kulanchoe blossfeldiana cv. Tom Thumb, CAM can be induced by short-day treatment or water deficiency stress. From young leaves of well-watered and water-stressed individuals of this plant, cDNA clones coding for a partial sequence of the key enzyme of CAM, phosphoenolpyruvate carboxylase, were isolated after transcription of mRNA. cDNA polymorphism was established by enzyme restriction profiles and sequencing data. Four PEPC isogenes could be shown to exist in K. blossfeldiana forming two gene pairs, with 95%-98% homology inside and only 75% between the pairs. One cDNA sequence pair having a length of 1113 bp and an open reading frame of 371 AA was identified as PEPC isoform specific for the C3 state, whereas the pair having a length of 1116 bp and an open reading frame of 372 AA could be attributed to the CAM state. These results were confirmed by Southern Blot hybridization.