BACKGROUND: Increasing developments in nanotechnology in recent years have raised the possible risk of release of nanoparticles into the aquatic environment, which could adversely affect organisms. Silver nanoparticles possess antibacterial activities and constitute almost 56% of total manufactured nanoparticles which are currently in use. OBJECTIVES: The aim of this study was to evaluate the effects of colloidal silver nanoparticles on bacterial flora of rainbow trout gut. METHODS: The Induced Coupled Plasma (ICP), Zetasizer and Transmission Electron Microscope (TEM) methods were used in order to ensure the quality of silver nanoparticles. 90 Fish (25±0.5 g) in Faculty of Marine Sciences were exposed to zero and 1 mg/L of silver nanoparticles over 14 days. Total viable bacterial counts and psycrotrophic bacteria, entrobacteriaceae and lactic acid bacteria counts of fish,s gut and total viable bacterial counts and psycrotrophic bacteria counts of water were determined. RESULTS: There was not any effect of nonoparticles on the bacterial counts (lactic acid bacteria, psycrotrophic and enterobacteriaceae) of the fish,s gut and bacterial counts (psycrotrophic and total viable of the water. However, there were significant differences between control and 1 ppm treatments based on the size and color of psycrotrophic colonies. Furthermore, the psycrotrophic bacterial counts of fish,s gut in the 1 ppm treatment showed significant increase (6.49±0.02 log cfu/g) compared to the control group (5.72 ± 0.17 log cfu/g) (p<0.05). CONCLUSIONS: The psycrotrophic bacteria represented the most sensitivity to nanoparticles among all studied bacterial groups (p<0.05); so, changes in the number of these bacteria may be considered as an indicator in monitoring the release and presence of silver nanoparticles in aquatic ecosystems.