The effects of food concentration and predation on the regulation of tropical planktonic ciliate communities were assessed. Ciliate abundances, biomasses, and growth rates were estimated following nutrient and predator manipulations at 2 tropical sites of different trophic status. Initial ciliate abundances and biomasses were higher at the eutrophic Kingston Harbour station than at the oligotrophic Discovery Bay station. Following in situ manipulations at both sites, it was found that the ciliates at Discovery Bay were food regulated while the ciliates of Kingston Harbour were regulated by their predators. Size fractionation incubations at Kingston Harbour revealed that the greatest predation pressures were acting upon the ciliates in the 600 μm treatments and were lowest upon the ciliates in the 45 μm treatments. Thus, this tropical ciliate community is apparently regulated by bottom-up control at the oligotrophic Discovery Bay site, and by top-down control at the eutrophic Kingston Harbour site.