A pULB113 (RP4::mini-Mu cts) plasmid was used to generate a library of prime plasmids carrying fragments of the Methylobacillus flagellatum genome. The genes carried by these prime plasmids were identified by complementation after transfer to suitably marked Escherichia coli and Pseudomonas aeruginosa strains. The hybrid plasmids were used for complementation mapping with a range of E. coli, M. flagellatum, and P. aeruginosa mutants. A preliminary map of the M. flagellatum genome section with seven groups of linked markers was obtained. Three of seven groups contain an overlapping sequence of cloned genes and can be considered as one large group of linked genes. A high-frequency-of-recombination donor of M. flagellatum (strain MFK64) mobilized the chromosome in a polarized manner from a single transfer origin. The donor was used to construct a time-of-entry map of the M. flagellatum chromosome. This was achieved by determining the time of entry of six randomly dispersed markers, four of which are included in known groups of linked markers. The linear map of M. flagellatum reported here consists of 44 markers.