The various species of the aquatic fern genus Marsilea grow in India throughout the year, braving the vagaries of climate and thus indicating their wide tolerance to different factors of climate, of which temperature is one of the most important. Different species seem to have different temperature tolerance ranges. Marsilea quadrifolia grows in Kashmir (Gupta, 1962) where the winter temperature goes far below freezing, whereas M. aegyptiaca occurs even on bare rocks and survives summer temperatures exceeding 50?C in Jodhpur (Gopal, 1968). It is true that water plants are at an advantage inasmuch as the fluctuations in water temperature are not so wide, but this does not altogether apply to amphibious plants like Marsilea, which pass a part of their life under terrestrial conditions during the dry season. These considerations led to some experiments on five Indian species of Marsilea: M. aegyptiaca, M. maheshwzarii, 1M. minuta, M. quadrifolia, and M. rajasthanensis. Their responses to different temperature treatments are reported below. In all the experiments five-internode pieces of rhizome taken from clonal cultures were transplanted to earthen pots. After three days, when the plants had become well established, they were subjected to the various experimental treatments. In the first set of experiments, the potted plants were kept at three different temperatures: (i) at a constant 15?C in a phytotron chamber, (ii) in an open garden where the temperature fluctuated between 25?C at night and 40?C during the day, and (iii) in a phytotron chamber at 15?C during the night and in the open at 38-40?C during the day. In the first two treatments normal growth occurred, but the total dry matter accumulation was more than twice as much at the higher daytime temperature than at continuous 15?C. There was almost no growth in the third treatment where the diurnal temperature was about 25?C hotterthan 150