This study has demonstrated the overall importance of zooplankton as a food resource for fish belonging to three contrasting trophic categories inhabiting isolated waterholes of an Australian dryland floodplain river. The contribution of zooplankton varied from 99.9% in the micro-carnivore Ambassis agassizii to 47% in the omnivore Leiopotherapon unicolor and 13.7% in the detritivore Nematalosa erebi. Moreover, zooplankton formed the basis of the diet of all species when young and relatively small, with all three species showing distinct shifts in diet composition across size classes. These trends varied across seasons and/or among waterhole locations. Since each waterhole presented a different pattern of flooding and subsequent drying, some being flooded and others not (Medeiros, 2005), such seasonal and spatial variability is likely to have an impact on the patterns of resource availability was beyond the scope of this study.