Abstract:Investigation was made on species of aerobic hetertrophic bacteria, along with the presence of their plasmid DNA, from various sites where are clean or polluted by petrolium alkane,pesticide or mercury.Semicontinuous mixed culture in chemostat was carried out for the study on variation in percentage of bacteria containing plasmids. The results showed that in the chemostat niche 29 of 76 initial strains were found with plasmids. and all of 6 strains detected after 6 monthes running contained plasmids. The pe...