Retrieval by different mesh-sized plankton nets is compared, using the abundance and distribution patterns of cyclopoid copepods of the genus Oithona W. Baird, 1843 as a case study. Mesozooplankton was sampled during spring, summer, and autumn of 2000-2002 in the South China Sea. Plankton samples were collected by surface tows (5-0 m) and by oblique tows in three different depth layers (0-50 m, 0-100 m, and 0-150 m), with Norpac zooplankton nets of 333 and 100 mu m mesh size. The oithonid density and diversity in the plankton samples collected through a 100 mu m mesh was about one order of magnitude higher than those collected through a 333 mu m mesh. The mean occurrence frequency of Oithona spp. in the plankton samples collected through a 100 mu m mesh net was > 85% and > 60% through a 333 mu m mesh net, with a mean total density of 45.86 +/- 206.97 m(-3) for the 100 mu m mesh net compared to 2.28 +/- 21.94 m(-3) for the 333 mu m mesh net [which indicates the enormous patchiness in occurrence of these copepods]. The density of smaller mesozooplankton, as evidenced by Oithona spp., has thus been underestimated by previous researchers using larger meshed nets. Samples collected through a 100 mu m mesh net demonstrated the numerical importance of the genus Oithona and its distinct seasonal and interannual patterns of abundance and distribution in the South China Sea.