Abstract: This study was done to investigate of new mercury resistant bacteria in coastal waters of Mahshahr area which the greatest petrochemical chlor-alkali unit of Iran located there. Water and sediment samples were taken and cultured on specific media in laboratory. Traditional biochemical and modern molecular approaches were used for identification. Mercury toxicity was measured via Minimal Inhibitory Concentration method. New observation of a coryneform, the Exiguobacterium sp. AT1b was occurred and identified based on 16S rRNA gene homology. Sequence matching was 99% and resistance to mercuric chloride were 75 and 50 ppm in different media. Optimal growth conditions was studied and temperature range plus with optimum pH were determined. Wide range of temperature tolerance and other conditions such as salinity and pH variation tolerance were observed. Agitation was the least affecting parameter on the growth based on optical density measured and analysis of variance results. The Exiguobacterium sp. AT1b is one of the best bacteria suggested for environmental studies and simulations.