Background and objective: Because of the health hazards associated with swimming pools, it is essential to monitor their water quality to insure safety of water and compliance with standards. This study aimed to survey water microbial quality of swimming pools in Gorgan during 2009 in terms of free chlorine residual and microbial indicators including Total Coliform, Fecal Coliform, Pseudomonas aeruginosa, and Fecal Streptococcus. Methods: In a cross-sectional study, a total number of 209 samples were collected from all swimming pools in 2009. The samples were shipped to a laboratory (maintaining cold chain) to determine microbial quality. In addition, on-site measurements of free chlorine levels were made using special Kits based on the DPD colorimetric method. Results: The Results showed that in 2009, chlorine levels in swimming pool waters in 25.84%, 74.16%, 67.94%, and 41.15% of samples, respectively, were 0, >0, >0.6 mg/l, and in an optimum range of.1-3 mg/l. 14.35%, 11.96%, 10.05%, and 8.13% of samples were contaminated to Total Coliform, Fecal Coliform, Pseudomonas aeruginosa, and Fecal Streptococcus, respectively. A significant relationship was found between microbial quality and free chlorine levels. Conclusion: Some levels of microbial contamination were observed in swimming pools. Almost all the contaminations were occurred in those samples with no free chlorine residual. The residual chlorine yielded significant improvement in water quality of swimming pools in Gorgan. Since free chlorine was equal to zero in ~ 25% of samples, attention should be paid to keep free chlorine levels in allowable ranges.