Abstract: The net photosynthesis of Thuidium cymbifolium and Chrysocladium retrorsum, two species of wintering host mosses for gullaphids,and its response to light,temperature and water content were measured with CI 301PS(CID Inc.USA)both in winter and spring.The photosynthetic capacity of Thuidium cymbifolium and Chrysocladium retrorsum was about 141 and 117μmolCO₂kg^-1 dw·s^-1,respectively,and trended to increase from winter to spring.The light saturation point of these two mosses at 800～900μmol·m^-2·s^-1 was much higher than that of many other mosses,and the compensation point ranged from 40 to 50μmol·m^-2·s^-1.The temperature response curves of these two mosses were similar,with optium temperature ranging from²5 tO₃6℃ in spring,and from²0 tO₃0℃ in winter.When the temperature was below the freezing point(-15 to 0℃),they both maintained a distinct net photosynthesis,with the optimum water content ranging from²00 tO₃00(400)%dw.The photosynthesis started to be restrained evidently,when the water content declined to about 150%dw.The gas exchange ceased or became negative,when the water content was as low as 40～50%dw.It can be inferred that these two species might be both poikilothermal and poikilohydric organisms,but the resistibility of T.cymbifolium to intense light and high temperature was higher than that of C.retrorsum.