Species composition and distribution of freshwater crustacean plankton in 12 Thailand water bodies were determined from samples collected from June, 1972 through March, 1976. Forty-four taxa of copepods and cladocerans were observed. The cyclopoid copepod and cladoceran components of Thai zooplankton are primarily cosmopolitan or cosmotropical. Calanoid copepods appear to be more limited in distribution and further sampling may reveal new endemic species. No distinct correlations were evident between species composition and distribution and environmental variables (e.g., age of water body, depth, geographical location and water quality features). However, it is recommended that in future investigations quantitative sample be obtained from each locality at the same time of the year so that data gathered is more comparable. The average number of species in all water bodies ranged from a winter low of 4.6 to a high of 13.0 during the rainy season. Species that occurred most frequently during the rainy season were Microcyclops varicans and Diaphanosoma sarsi. The most prevalent species during summer were Ceriodaphnia cornuta, Moina micrura, Bosminopsis deitseri and Chydorus ventricosus. Some species such as Mesocyclops leuckarti and Diaphanosoma excisum were predominant year-round.