This study aimed to: compare the adhesion of bacteria by using Brain heart infusion agar plates in class III modification I maxillary partially edentulous patients wearing removable partial dentures constructed from two different injectable thermoplastic materials PEEK and BRE.FLEX (second edition). : twenty-six patients having Kennedy class III modification I Maxillary partially edentulous ridges with fully dentate mandibular arch were divided into two equal groups to fabricate the thermoplastic materials. The first group received Bre.Flex material. The second group received PEEK material. Within the Bre.Flex group, the bacterial growth has increased as the follow up period increased. Within the PEEK group, the bacterial count has increased as the follow up period increased. The PEEK group showed a greater bacterial growth than the Bre.Flex group in all the follow up periods and the difference was statistically significant.The result of the study showed The PEEK group showed a greater bacterial growth than the Bre.Flex group in all the follow up periods and the difference was statistically significant.