<p>The ingestion of two size classes of natural particulate matter (PM) and the uptake of the associated nitrogen by four species of scleractinian corals was measured using the stable isotopic tracer N-15. PM collected in sediment traps was split into &lt; 63 and &gt; 105 mum size fractions and labeled with (N-15-NH4)(2)SO4. Siderastrea radians,Montastrea franksi, Diploria strigosa, and Madracis mirabiliswere incubated in flow chambers with the labeled PM in suspension (&lt; 63 &mu;m), or deposited onto coral surfaces (&gt; 105 mum). Ingestion was detected for all four species (98-600 mug Dry wt. cm(-2) h(-1)), but only for D. strigosa was any difference detected between suspended and deposited PM. Only the three mounding species, S. radians, M. franksi, and D. strigosa showed uptake of suspended and deposited particulate nitrogen (PN); whereas, the branched coral M. mirabilis had no measurable PN uptake. Only coral host tissues were enriched with N-15, with no tracer detected in the symbiotic zooxanthellae. Uptake rates ranged from as low as 0.80 mug PN cm(-2) h(-1) in S. radians to as high as 13 mug PN cm(-2) h(-1) in M. franksi. M. franksi had significantly higher uptake rates than S. radians (ANOVA, p &lt; 0.05), while D. strigosa had a statistically similar uptake rate compared to both species. These results are the first to compare scleractinian ingestion of nitrogen associated with suspended and deposited particulate matter, and demonstrate that the use of PM as a nitrogen source varies with species and colony morphology.</p>