Research was carried out in Udorn Thani beginning in September, 1975, in order to elucidate the effects of seasonal changes on the aquatic community of a rice field. Physical and chemical changes that occur during the course of a year were recorded and related to the structure and activity of the aquatic biological com- munity. A rice field was selected that is typical of the countless habitats of this kind that are found in Northeastern Thailand (Fig. 1). For a variety of reasons these habitats and their location are of special interest (Fig. 2). Figure 1. The rice field investigated, north of the province capital, Udorn Thani, Thailand. Northeastern Thailand and a small part of Laos comprise the middle Mekong Valley. Its geographic situation and climatic conditions set this area apart as a dis- tinct sub-region of Southeast Asia. It is unfortunate that sufficient biological research has not yet been carried out to delimit the SUb-regions according to their fauna and flora and to fully describe the various ecosystems that occur in them. The middle Mekong Valley is one of the most neglected areas of the region, and there is no comprehensive literature on its biota. Very little ecological information is available on many of the species that abound there.