Biofilms are communities of microorganisms that are attached to a specific surface and are encased in an extracellular matrix that they produce themselves. These structures create complex microhabitats. allowing for intense biological and chemical interactions among different microbial species. In the context of aquatic ecotoxicology. biofilm has an extraordinary ability to absorb and accumulate pollutants. including heavy metals. so it has the potential to be used as a biological indicator that is sensitive to changes in environmental quality. This study aims to analyze and compare the concentration of heavy metals. especially hexavalent chromium (Cr(VI)). in biofilms. sediments. and water in the Manyar River. Gresik. Indonesia. in two different hydrological periods. namely the dry season and the rainy season. Sampling was carried out at three stations representing different environmental characteristics along the river flow. The physico-chemical parameters measured include temperature. depth. flow velocity. pH. precipitation. and dissolved oxygen (DO) levels. Heavy metal concentration analysis uses the Atomic Absorption Spectrophotometry (AAS) method to obtain precise and reliable results. The data showed that the highest heavy metal content of Cr(VI) was found at Station I which was close to the source of industrial activity. while the lowest value was found at Station III which was further away from the area. All measured Cr(VI) concentration values exceeded the national river water quality standard threshold of 0.5 mg/L. These results prove that biofilm has a heavy metal accumulation capacity of up to 66-98 times higher when compared to water and 67-100 times when compared to sediment. confirming its role as an efficient natural bioaccumulation agent. Thus. biofilm can be used as an effective and sustainable biomonitor to detect and evaluate the level of heavy metal pollution in aquatic ecosystems. These findings have the potential to be the basis for the development of environmentally friendly and low-cost bioindicator-based environmental monitoring systems.