Although the taxonomy of oligotrich ciliates has been widely investigated,yet the species diversity remains poorly known.We newly designed a pair of oligotrich-specific LSU rDNA primers covering the 600-bp D1/D2 region,and it was effective for detecting oligotrich species.Using the primers,we constructed the cloning libraries to investigate the species diversity of oligotrichs in the north coastal waters of the South China Sea.In total,165 oligotrich sequences were obtained from five widely separated sampling sites.Sixty operational taxonomic units (OTUs) were obtained at 99％ similarity threshold,and low-abundance OTUs with no more than two sequences contributed most of these (about 78％).Our findings are consistent with previous morphological studies,Strombidium was found the most abundant and widely distributed genus in this area.In addition,the BLAST search in the NCBI database resulted in 95％ OTUs matching with named oligotrich species in similarity below 99％.Therefore,oligotrich morphospecies diversity has been underestimated as low-abundance species,and the LSU rDNA oligotrich sequence database needs to be better promoted.