Zooplankton were sampled quantitatively in the lower Orinoco River and three major tributaries (Apure, Caura, Caroni). Dominant taxa were euplanktonic, which suggests origin from standing waters. In the Orinoco, densities for rotifers (mean, 7.6 ind. liter -I) were greater than those for cladocerans (0.9 ind. liter-I) or copepods (1.2 ind. liter-'), but cladocerans accounted for more than half oftotal biomass (0.5 kg C liter -I). Average tributary contributions to zooplankton transport in the lower Orinoco (873 kg C d-l) were proportional to their average discharges. Extensive fringing floodplain along the lower Orinoco contributed very little to total transport. At low water in the Orinoco, when there was no contact with the floodplain, the transport of copepods (mainly nauplii) and Bosmina increased downstream by more than could be ascribed to additions from the tributaries or to reproduction in transit. These zooplankton appear to originate along the river margin, within the channel, and arc probably significant as an energy input for food webs in the channel and as an inoculum to floodplain habitats at the time of inundation.