We present an ecological study of the Cladocera in a shallow tropical freshwater lake (Thale-Noi, Thailand) using activity traps in different habitats. Depth and pH shaped the communities regardless of macrophyte composition and overall densities were lower than reported from temperate lakes. Species composition and densities varied monthly, yet at any time maximally 22 cladoceran species were able to coexist in a niche of the dimensions studied here, the majority Chydoridae.