Aims: The aim of the present study was to describe an aerobic, mesophilic and heterotrophic bacterium, designated RA26, able to produce a medium-chain-length polyhydroxyalkanoate (PHA). It was isolated from a French Polynesian bacterial mat located in the atoll of Rangiroa. Methods and Results: This micro-organism, on the basis of the phenotypical features and genotypic investigations can be clearly assigned to the Pseudomonas genus and the name of Pseudomonas guezennei is proposed. Optimal growth occurs between 33 and 37 degrees C, at a pH between 6 4 and 7 1 and at ionic strength of 15 g 1(-1) of sea salts. The G+C content of DNA is 63.2%. Under laboratory conditions, this bacterium produced a novel, medium-chain-length PHA, mainly composed of 3-hydroxydecanaote ( 64 mol.%) and 3-hydroxyoctanoate ( 24 mol.%) ( GC-MS, NMR) from a single nonrelated carbon substrate, i.e. glucose. Conclusions: The bacterium P. guezennei produces a novel PHA mcl with elastomeric properties. Significance and Impact of the Study: PHAs share physical and material properties that recommend them for application in various areas, and are considered as an alternative to nonbiodegradable plastics produced from fossil oils. In this study, we describe a new bacteria with the capability to synthesize a novel PHA with promising biotechnological applications.