The broad-host-range IncP plasmid RP1 could not be transferred by conjugation from Escherichia coli to Pseudomonas fluorescens strain CHA0. However, this conjugative transfer was possible with RP1 derivatives which had large deletions extending from the primase gene towards the Tra-2 region, thus lacking the kanamycin resistance gene and IS21. Such RP1 deletion derivatives permitted IncP cosmid mobilization to P. fluorescens CHA0 and could be used as vectors for transposon mutagenesis with a newly constructed Tn5 derivative (carrying kanamycin and mercury resistance determinants) in strain CHA0 and another P. fluorescens soil isolate, strain S9.