The present study aimed to study the fitness between a trans-conjugant (recombinant strain) of Pseudomonas aeruginosa bacteria with its parents after transferring genetic material by conjugation mechanism. Whereas, environmental fitness expresses the interaction of an organism with its environment therefore it is considered a good indicator for the assessment of genetically engineered microorganisms (GEM) released into nature. Incubation time was carried out in vitro and incubating time in situ in Mowas River Zagazig city during winter and summer. Accordingly, the fitness of the parents and the recombinant strain was studied. The three strains of Pseudomonas aeruginosa (PAO1, MAM2 and PU21) were tested on chloramphenicol and tetracycline. Strain MAM2 was resistant to chloramphenicol 1200 μg/ml while was sensitive to tetracycline and has been used as the recipient. While strain PAO1 was resistant to tetracycline 200 μg/ml and was sensitive to chloramphenicol and has been used as the donor. Results proofed the presence of the plasmid in the donor and trans conjugant strains. The donor was treated with acridine orange to match the results obtained with the results at the molecular level. It was observed that bacterial fitness continued for up to 35 days in vitro, while in situ during the summer it did not last at the site for only 21 days. While it lasted 28 days during the summer. So, the risks that may be caused by releasing the genetically modified microorganisms into environments have been canceled. In addition to its ability to preserve the new genetic material, it may be able to transfer this new genetic material to other strains and species that may be live in the same ecosystem, as it is largely stable in the environment. In genetically modified microorganisms that are added to the environments for agricultural uses such as increasing soil fertility (bio-fertilizer) or biodegradation for a harmful substance such as pesticides, the soil must be re-inoculated in winter every 21 days and during summer every 28 days due to the loss of the plasmid, which carries some important genes.