A cryptic plasmid, pSM103mini, was found in polyethylene-glycol degrading bacterium Sphingopyxis macrogoltabida, strain 103. The plasmid was 4,328-bp long and its GC content was 57.5%. It contained four open reading frames, but only two of them showed significant similarity to reported proteins. ORF3 and ORF4 were assumed to encode resolvase and replication protein (RepA) respectively. Downstream of ORF4 we found complex repeat sequences. These together with ORF3 and 4 were necessary and sufficient for plasmid maintenance in strain 103, as analyzed by constructing deletion plasmids. The pHSG398-fused plasmid (pHSG-SM103mini) functioned as a shuttle vector between strain 103 and Escherichia coli. The plasmid constructed was maintained in strain 103 and its close relative, S. macrogoltabida strain 203, but not efficiently in PEG-degrading S. terrae.