BACKGROUND: Microbial quality of drinking water is very important in animal health. OBJECTIVES: The present survey was conducted to evaluate the microbial quality of cows' drinking water in dairy farms in Mashhad suburb, Iran, to find out defects in this field. METHODS: Water samples were taken from 30 farms and 4 sites including: Water tanks,  inlet and outlet  of bovine drinking troughs and calves’ water buckets in fall, 2018. The samples were put in sterile falcons, with keeping of the cold chain and immediately sent to the microbiological laboratory. Counting of fecal Streptococcus and Coliform in the specimens were performed by using pour plate and most probable number (MPN) methods, respectively.The contamination frequency of the samples to fecal Coliform and Escherichia coli were determined by using specific biochemical tests. The positive specimens in terms of E. coli were also detected for the presence of serotype O157: H7 by using PCR technique. RESULTS: According to the sampling sites, the contamination frequency with fecal Coliform and Strepto- coccus were recorded 30-100% and 20- 96.67%, respectively. The most frequency of contamination were observed in outlet of bovine drinking troughs. In 3.33% of samples, serotype O157 : H7 and in 6.67% of samples, undefined H7 serotype were diagnosed. There was no statistical significant difference in the level of bacterial contamination of drinking water due to the geographical location of farms (p > 0.05). CONCLUSIONS:It was concluded that except for a limited number of farms, water tanks are relatively safe, while it is necessary to pay particular attention to the high contamination of outlet of bovine drinking troughs and water buckets of calves.