The penultimate instar (tritonymph) of Naiadacarus arboricola, a saprophagus mite restricted to water-filled treeholes, is the most tolerant of all instars to adverse environmental conditions when tested in the laboratory (i.e., freezing, high temperatures, sta.rvation). Also, when reared on inadequate food sources or at low temperature, development usually proceeds only to the tritonymphal stage. Field observations on natural populations indicate that during a dearth of food and also during winter the tritonymphal instar predominates. When adequate environmental conditions return, the tritonymphs transform to adults almost simultaneously. The tritonymphal stage thus phases the population in response to cooling temperatures and/or lack of food, re:mlting (by synchronous molting) in a population of young, highly reproductive adults when adequate conditions return to the treehole. Such a population, through its high reproductive potential, can best compete with other saprophagus organisms for the available food supply.