Protists (microbial eukaryotes) are indispensable members of the marine microbial food web.In recent years,organisms living in the deep sea (> 1000 m water depth) have increasingly become the focus of research; however,studies on protistan assem-blages are relatively scarce compared with their prokaryotic counterparts.In the present study,high-throughput sequencing of the hypervariable V9 region of the 18S rRNA gene was used to explore the community composition of protists in bathypelagic waters of the South China Sea.Based on the analysis of the alpha and beta diversities of 14 samples,we discovered: 1) members belonging to Rhizaria,Alveolata,and Excavata were the dominant groups in terms of both relative sequence abundance and operational taxonomic unit (OTU) richness in all samples,although their relative contributions differed among different samples; 2) cluster analysis showed that the distribution of protistan assemblages was related neither to the sampling location nor to the water depth,and other environ-mental factors might have caused the differences among the communities; 3) phototrophs,including members of the Bacillariophyta,Bolidophyceae,Dictyochophyceae,Prasinophyceae,and Prymnesiophyceae,were detected in all samples,which indicated their con-tributions to the downward transportation via the biological pump and the potential presence of phagotrophy of these phototrophic cells in the deep ocean.