Novel host-vector systems have been developed for gene cloning in the metabolically versatile bacterial genus Pseudomonas. We found that a new Pseudomonas strain, Pseudomonas flavida IF-4, isolated from soil, carried two small cryptic plasmids, named pNI10 and pNI20. They were multi-copy, but not self-transmissible, and the genome size was 3. 7 kb for pNI10 and 2. 9 kb for pNI20. Several types of cloning vectors containing a kanamycin or streptomycin resistance (Km^r or Sm^r) gene were constructed from pNI10 and pNI20. These plasmid vectors were efficiently transformed into several strains of Pseudomonas at a frequency up to 4×10⁵ transformants per 1 μg plasmid DNA by the usual competent cell method. The vectors derived from pNI10 replicated not only in Pseudomonas but also in some other Gram-negative enteric bacteria such as Escherichia coli, Enterobacter aerogenes, and Proteus mirabilis.