Comparaison des effets des températures constantes ou variables sur la durée de déeveloppement de Damaeus onustus (Acarina: Oribatei). - This paper deals with comparison of the developmental period of ’eggs’ of Damaeus onustus at a given mean temperature applied either constantly either variabily with Jess or well marked amplitudes. The development of 71 eggs has been studied from September 73 to April 74. For each egg the real developmental period (from oviposition to hatching), the corresponding mean temperature and the thermie variations to which the animais were exposed, were recorded. The mean temperature enables to predict the developmental period (dt11) if the temperature remains constant, thanks to the relation between constant temperature and developmental period established in a previous work. The comparison of the observed period (dobs) with the theoretical one (dt1,) shows high variations ranging from 4 to 300 %: the observed period is practically always Jess at variable temperature. The interpretation of the results brings the author to underline the essential role of thermie variations in this phenomenon. The influence of this variations is, however, complex because too large variations including temperatures near o°C delay hatching time. I t is possible that the prelarval moulting is inhibited by this kind of variations even if the development is completely made. This stage step in development of the animal undergoing the variations also appears to be an important factor.