A 1.5kb cryptic plasmid was isolated from Helicobacter pylori. Low‐stringency hybridization analysis using this plasmid as a DNA probe revealed base sequence homology with other plasmids in this species. Nucleotide sequence analysis identified an open reading frame encoding a putative polypeptide of 25 kDa. This protein showed marked amino acid sequence similarity to replication‐initiation proteins commonly found in small plasmids endogenous to Gram‐positive bacteria which replicate by the ‘rolling‐circle’ mechanism. Sequence motifs corresponding to the origins‐of‐replication consensus sequences were found on this cryptic plasmid. DNA and oligonucleotide probes to these plasmid replication sequences were used in hybridization analysis to identify similar sequences in other H. pylori plasmids. We believe this is the first plasmid isolated from a Gram‐negative bacterium to show replication determinants characteristic of the ‘rolling‐circle’ group of plasmids from Gram‐positive bacteria. The cloned plasmid will be used to develop a shuttle‐vector for H. pylori.