In recent years wastewater treatment using microbial processes are very common. Activated sludge process using bacteria is the most widely used technology for municipal and industrial wastewater treatment. In present study we detected Nitrosomonas sp., Pseudomonas sp., Proteus inconstans, Pseudomonas fluorescence, and Pseudomonas aeruginosa in activated sludge of municipal wastewater in cold climate regions of Iran. Total of 35 municipal wastewater specimens were collected from different regions of Chaharmahal Va Bakhtiari province in cold months and genomic DNA was extracted using kit. PCR reaction was performed using specific oligonucleotide primers for amplification of each genes and amplified products visualized on 1% agarose gel electrophoresis. From 35 municipal wastewater samples 22 (62.86%), 25 (71.43%), 9 (25.71%), 13 (37.14%), and 17 (48.57%) specimens were positive for Nitrosomonas sp., Pseudomonas sp., Proteus inconstans, Pseudomonas fluorescence, and Pseudomonas aeruginosa, respectively. These results showed that Pseudomonas sp. and Nitrosomonas sp. are more frequent bacterial species in activated sludge of municipal wastewater of cold climate regions. These findings demonstrated that growth controls of these bacteria are important in biological treatment processes of activated sludge and useful for decrease of biochemical oxygen demand (BOD) in municipal wastewater of cold regions.