Tropocyclops bopingi n.sp. is a 0.4 mm cyclopoid found in Liuxi He Reservoir, Guangdong, South China. It is probably widespread in South China, and appears in the Fauna Sinica under the name T. parvus. True T. parvus is a Central American species that is here redescribed from its type locality, Lake Petén Itzá in Guatemala. T. bopingi is related to T. tenellus from the limnetic zone of Lake Tanganyika and other lakes in Central Africa, and all of them live under climatic conditions that are transitory between subtropical and tropical. T. tenellus is even smaller and more buoyant than T. bopingi and, at least in Lake Tanganyika, it tends to occur closer to the water surface than other zooplankton during the daytime. It does not carry egg balls, but extrudes eggs one at a time. T. tenellus and T. bopingi appear morphologically closer to each other than to T. parvus, but all three are ecologically similar: extremely small, pelagic species that presumably feed on small particles in the water. I hypothesize that environmental conditions in subtropical-tropical transition zones combine with predation and competitive pressure to offer a selective advantage to such omnivorous species in the open water of large lakes or reservoirs.