Citarum River is the largest natural stream in West Java, Indonesia, flowing across an area of 6614 km 2 . About 3000 industries discharge their wastewater into the stream, affecting almost 19 million people who live along the river. Considering the perseverance and the prospective toxicity of microplastics (MPs), investigating their concentrations in this river is critical to help illustrate the exposure of the risks to the residents of the area and beyond. This study was focused on identifying the MPs concentrations in the water, sediment, and milkfish (Chanos chanos). A volume-reduce method by using manta trawl was used to take water samples. Sediment and milkfish samples were taken using a grab sampling method. Digestion of fish was using Fenton oxidation method according to weighted ratio (1:5) and H 2 O 2 30% (w/v). The average MPs concentration in the river was 0.0574 ± 0.025 particles/m 3 ; in the seawater ponds 3.000 ± 2.645 particles/L; and in the mixed-water ponds, where the water from the river and the sea were mixed, 0.666 ± 0.577 particles/L. The average MPs concentration in the sediment of Citarum River was 16.666 ± 0.577 particles/100 g; in the seawater ponds 13.335 ± 1.527 particles/100 g; in the mixed-water ponds

Microplastics in aquatic environment can possibly enter into the aquatic organism, so this study aims to identify the microplastic concentrations in water, sediment, and milkfish. The samples are taken in downstream Citarum river at Muara Gembong. Water samples were taken using a manta trawl pulled by a boat while sediment and milkfish samples were taken using the grab sampling method. Organic removal and cleaning was conducted by using the Fenton Oxidation method which use Fe and H2O2. The average of microplastic concentrations in river water, seawater ponds, mixed water, and seawater consecutively are 0.0574 ± 0.025 particles/m3, 3.000 ± 2.645 particles/L, 0.666 ± 0.577 particles/L and 1.333 ± 1.155 particles/L. The average of microplastic concentrations in sediment sea water pond, sediment mixed water pond and sea sediment samples consecutively are 3.666 ± 0.577 particles/20g, 2.667 ± 1.527 particles/20g, 2.333 ± 0.577 particles/20g and 0.667 ± 1.154 particle/20g. The average of microplastic concentrations in the gut and gills of milkfish in sea ponds and mixed ponds consecutively are 2.333 ± 2.266 particles/fish and 2.222 ± 3.768 particles/fish. The average of microplastic concentrations in milkfish tissues in the sea and mixed water ponds are 1.333 ± 1.000 particles/fish and 1.111 ± 1.167 particles/fish.