Six bacterial isolates belonging to the genus Pseudomonas were selected for their high ability to utilize crude oil to be used throughout this study. Utilization of different hydrocarbon compounds (aliphatic, aromatic, alicyclic and furan) as a sole source of carbon and energy for these isolates was studied. The results showed a diverge ability of hydrocarbon utilization by these isolates, and two isolates (P. aeruginosa RB19 and Pseudomonas sp. RB29) were able to utilize all the tested compounds expcept tetrahydrofuran, sodium toluate and trans-l,2-diphenylethylene. Conjugation experiments were done to study the role of plasmid isolates, in utilization of the hydrocarbon compounds. Conjugation between P. aeruginosa RB27 and E. coli MM294 resulted in two types of transconjugants. The first type of E. coli was able to utilize benzoate and toluate, and contained large and small plasmids. The second type of E. coli was able utilize the same hydrocarbon compounds in addition to octane and contained both plasmids. The large plasmid was responsible for rjienzoate and toluate utilization and it is a self-transmissible plasmid. The small plasmid had no role in hydrocarbon utilization because of its small size. Conjugation between Pseudomonas sp. RB29 and E. coli MM294 resulted in transconjugants able to utilize benzoate, salicylic acid and octane and contained large and small plasmids. The large plasmid was responsible for benzoate, salicylic acid and octane utilization and it is a self-transmissible plasmid. Conjugation between P. fluorescens RB16 and E. coli MM294 resulted in transconjugants able to utilize benzoate and octane and contained large and small plasmids. The large plasmid was responsible for the benzoate and octane utilization and it is a self-transmissible. Curing experiments were performed for P. aeruginosa RB31 and Pseudomonas sp. RB7 (isolates that were failed in conjugation with E. coli MM294). Results showed that some colonies (6%) of RB7 have lost their ability to utilize benzoate, toluate and salicylic acid and lost their two plasmids. The large one is responsible for the utilization of tfifese compounds, while small plasmid may have a regulatory role in utilization process. In case of P. aeruginosa RB31 some colonies (5%) had lost their ability to utilize benzoate and toluate and lost their two plasmids. The large one is responsible for these hydrocarbons utilization and the small one may have a regulatory role in utilization process. Conjugation between P. aeruginosa RB19 and P. aeruginosa RB27 was made in an attempt to improve bacterial strains able to utilize a wide range of hydrocarbon compounds. Results showed that the conjugation process was successful. Number of transconjugants were' selected and tested for their ability to utilize hydrocarbon compounds. It was found that all transconjugants were able to utilize toluate, and some of these transconjugants were able to utilize salicylic acid. This indicated that P. aeruginosa RB27 was the recipient bacterium when some cells of this bacterium received pSR202 plasmid which responsible for salicylic acid and benzoate utilization from P. aeruginosa RB19.