Summer drought and winter flood introduce a variable into an otherwise constant cave environment. This alternation imposes recurrent quiescent and active phases on such cave inhabitants as serpulid polychaete worms, plumatellids, and sponges. Flooding introduces food into the caves while regression of flood waters may isolate small groups of organisms into small pools for months at a time. Dispersal of animals from one cave to another may be brought about by flooding, and an illustration is given with a small cyprinid fish, how flooding may aid in the colonization of caves with fauna from the outside.