Phenology and growth characteristics of vulnerable species, Zannichellia palustris L., growing at dramage water course in reclaimed land Iya in Lake Nakaumi were observed in the field during 1997. Effect of water temperature, light intensity, salinity and leaf age on the photosynthetic activity was also examined by the laboratory experiments. The seedling showed an exponential growth and there observed c.600 shoots per plant after 40 days of the initiation, and fruiting started only within 2 weeks after the initiation. The plant height was 25 cm and it accumulated its biomass within a top 5 cm layer and light intensity decreased down to 0.46% through this dense canopy layer. Longevity of leaf was estimated to be 15.3 days on average by direct observation. Photosynthetic activity increased from the minimum at 15℃ to the maximum at 35℃ but showed negative value at 40℃. Effect of salinity on photosynthesis was distinct and the plant showed maximum value at 0 %o and it decreased with the increase of salinity up to 30 %o. The relation between leaf age and photosynthetic activity was also distinct and net photosynthes is of newly borne leaf was 3 times higher than that of 14 day old leaf.