Abstract:［Objective］ To identify and characterize an unknown microorganism causing contamination in several mammalian cell cultures.［Methods］This bacterium was identified by 16S rRNA sequencing and studied by DAPI and DiOC6 (3) staining，Gram staining，acid-fast staining，and electron microscopy． The isolated bacterium was also used to infect host cells to observe antibiotic effectiveness and its relationship with host cells.［Results］The 16S rRNA sequence analysis shows that this rod-shaped microorganism belongs to the family Caulobacteraceae，class Alphaproteobacteria，and was most closely related to Phenylobacterium zucineum HLK1T strain．The bacterium collected in the“swimming”stage was Gram staining negative，but Gram staining positive in the “sessile”stage．Under the electron microscope both flagellated and non-flagellated types were found． So far，no antibiotics were effective to inhibit this microorganism．The contamination with this bacterium frequently led to failed resuscitation of thawed cells． We found that the cells resuscitated with the used culture supernatants were increased in number by 3－4 folds as compared to those resuscitated with freshly prepared media． ［Conclusion］ Phenylobacterium may have a dimorphic life cycle including a swimming stage and a sessile stalked stage．