Systematic investigation of carbon and nitrogen isotopic compositions of suspended particulate matters, plants, soils, sediments, and bed rocks was conducted during 1993–1994 in the Lanyang‐Hsi watershed, which is a typical small Oceania river. The dramatic increase in particle concentration and changes in nitrogen isotopic composition from tributaries to main stem indicate a shift in the major sources of particulate matter from soil and plant detritus (biogenic) for tributaries to poorly weathered rock fragments (lithogenic) and soil for the main stem. Using a two end‐member mixing of nitrogen isotopes and the load‐runoff relationship, we estimated the mean yield of biogenic particulate nitrogen (PN) in this watershed to be 0.41 ± 0.19 g N m⁻² yr⁻¹, which comprised only 9% of the total PN yield. Assuming the mean atomic C/N ratio (13.6) of soil samples as that for the biogenic end‐member, we estimated the yield of biogenic particulate organic carbon (POC) to be 4.9 ± 2.2 g C m⁻² yr⁻¹. If this represents the natural POC yield in Oceania islands, more than half of the POC flux from these islands could have been humanly induced. The flux‐weighted mean δ¹⁵N and δ¹³C values of exported particles were +3.6‰ and −25.3‰. The δ¹³C value is similar to those observed elsewhere, whereas the δ¹⁵ value is considerably heavier, apparently attributable to the high contribution of lithogenic PN induced by human disturbance.