The suicide plasmid pVA838 carrying the operon fusion transposon Tn5-lac was used as a delivery system to introduce Tn5-lac into Pseudomonas sp. strain M114. Random, in vivo lac gene fusions were successfully isolated in a one-step conjugation approach with this vector system. Tn5-lac-containing exconjugants were recovered at a frequency of approximately 10(-7) per recipient. However, when the mating temperature was increased from the normal growth temperature (28 degrees C) to 34 degrees C, the frequency was increased to approximately 10(-4) per recipient. A number of in vivo lac gene fusions were isolated and characterized in strain M114, a potentially important bacterium for biological control purposes.