The environmental DNA (eDNA) method is being increasingly applied in various environments. Although eDNA undergoes rapid degradation in aqueous environments, it has been detected in streams up to 10 km downstream from its source. As environmental bacteria can uptake free DNA, transfer their genetic traits, and amplify, there is a potential risk that they, rather than a target aquatic species, could become a source of measured eDNA. This study examined whether bacteria with incorporated fish DNA could be such a source by investigating the detectability of fish DNA generated by bacteria inhabiting river water and riverbed sediment. We attempted to detect common carp (Cyprinus carpio) eDNA in stream water and sediment samples and the DNA of common carp produced by bacterial colonies (Escherichia coli, total coliform, and heterotrophic bacteria) cultured from the samples. The eDNA was detected in the environmental samples but the carp DNA from the targeted bacteria was rarely detected in both water and riverbed sediment samples. Our results suggest that the risk of bacterium-induced false positive detection for fish eDNA is negligible. The environmental DNA (eDNA) method has been increasingly applied to various organisms and environments. Although many eDNA studies have focused on detecting the presence or absence of species, the eDNA technique also shows considerable potential for inferring species abundance in lotic and lentic systems based on quantified copy numbers of target DNA fragments . Despite the potential advantages of the method, considerable challenges remain in terms of understanding the fate and dynamics of eDNA in water bodies. eDNA is genetic material that presents in environments such as water and soil originated from excreted cells or tissue including saliva, feces, urine. It has been shown that stream eDNA can be degraded by physical processes (e.g., advection and settling) as well as chemical and biological processes (e.g., hydrolysis by nucleases). The physical settling and subsequent resuspension of eDNA from a target taxon creates the risk of false positive detection. Consequently, even if a target taxon is detected with the eDNA technique, the distribution of that taxon remains unclear. Hence, a better understanding of stream eDNA dynamics is required for more reliable biological monitoring. You can share your ideas & comments further at : molecularbiology@scholarlymed.com molecularbiology@scholarlymed.com