In order to study caudal fin rot with emphasis on Aeromonas sp. and Pseudomonas sp. in Salmo trutta caspius from the salmonids propagation and breeding center of Shahid Bahonar of kelardasht region, One hundred and eighty brood stocks having fin damage symptoms were chosen. Two bacterial samples from each fish were cultured on Aeromonas and Pseudomonas specific media. To identify isolated bacterial colonies by API20E diagnostic system, samples obtained from bacterial cultures 18 to 24 h were prepared and were inoculated into the cupules of test strips. At the end of incubation, after addition of reagents if they are necessary, the results recorded on the results sheets and were analyzed by Apiwe software. The results of API20E diagnostic systems showed that Aeromonas genus bacteria including Aeromonas hydrophila with 100% frequency while the bacterial genus of Pseudomonas including Pseudomonas fluorescens, Pseudomonas aeruginosa and Pseudomonas putida with 36. 84, 31.57 and 21.05% frequency, respectively. Also 10.25% of Pseudomonas samples were unidentified.