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.