Extensive collections along the entire course of a small stream in a typical Panamanian rain-forest on Barro Colorado Island, Canal Zone, brought out the following significant facts: The moist stream bed is invaded by numerous land animals during the dry season, including such animals as termites. Some animals were found in all the pools of the stream but the majority showed some differential distribution. An analysis of environmental factors, including pH, dissolved O2, temperature, size and permanence of pools and of flow, type of bottom, amount and quality of bottom cover, made when differences were most marked, indicates that distribution of animal associations normally depends on the entire environment but again emphasizes the importance of the well-known factors of relative location and of character of substratum in the distribution of water animals.