Organically cultured rice fields were surveyed to compare the population density and the structure of microarthropod communities between chemically controlled and un-controlled fields. In rice fields, microarthropod communities were also compared among four treatments based on different input of fertilzer : 1)No fertilizer, 2)Chemical fertilizer(N-P2O5-K2O = 11 - 7 - 8 kg/10a, 3) Compost (5,000kg/10a), 4) Compost com-plemented with chemical N. Richness index of microarthropod communities in rice fields was greater in the chemically uncontrolled field than controlled field. Within field. the compost plot had greater value of richness index for microarthopod commuities than the non-fertilizer polt. A total of 9 aquatic insects species were identified in the water of organic farming rice field the population density was found to be higher in the compost plot than the other plot. The population of insect pests was found to be high in the late stage of plant whereas the population of insects was high in the early stage of rice plant.