Magnesium (Mg) alloys can be use as biodegradable medical devices, eliminating the need for a second operation for implant removal. An important feature on biomedical devices is to avoid the bacterial adhesion and subsequent biofilm formation that cause most of the implant-failures. The aim of this study was to analyze the differences on bacterial adhesion and biofilm development on Magnesium alloys (Mg-Al-Zn) modified by different transition metals; Tantalum, Niobium and Titanium. Nine oral bacterial strains (Aggregatibacter actinomycetemcomitans serotipe b, Actinomyces israelii, Campylobacter rectus, Eikenella corrodens, Fusobacterium nucleatum, Parvimonas micra, Porphyromonas gingivalis, Prevotella intermedia and Streptococcus sanguinis) were incubated on the different alloys and commercial medical grade stainless steel (AISI 316L) was used as a control. The initial bacterial adhesion was determined after 24 hours using a counting plate technique and the subsequent biofilm development at 1, 3, 7 days was studied using the Scanning Electron Microscopy. Significant differences were determined using t-test. The results showed that on the magnesium-alloys, the number of bacteria attached after 24 hours was two orders of magnitude lower than the stainless steel. On the other hand, bacterial colonies were not observed by electron microscopy in any of the days of incubation, even though in the control surface clear colonies and biofilm development were observed. This study showed that magnesium alloys inhibits the bacterial adhesion and the subsequent biofilm development.