Abstract: The ecological conditions of Nostoc flagelliforme in Yongdeng, Gansu and Alashanzuoqi, Neimenggu have been investigated in this paper. The water rhythm phenomenon in the growth of N. flagelliforme was discovered by us.This alga grow in desert steppes with continental climate. The mean annual temperature of such steppes should be 4.5—8℃ and the annual and daily differences in temperature are big and marked about 50℃ for the year and 15℃ for the day. The mean annual precipitation is only 201—290 mm and the mean annual relative humidity 45—58%. There are about 83 days with dewfall in one year. Therefore, the climate of such steppes is Very dry. Their soil shows a strong alkaline reaction, with pH values over 9.0 and contents of organic matters, nitrogen and phosphorus is very poor. However there is plenty of sunshine for the alga in such steppes, totalling 3000±300 hin one year.The growth of N. flagelliforme exhibits an obvious water rhythm phenomenon. From May to October, the dry alga absorbs quickly the water from rainfall or dewfall for its growth in a short duration. The water will beevaporated soon in the sunshine, and the alga will stop to grow and become dry again. The growth process of N. flagelliforme is a successive intermittent growth and accumulation in the rhythm of moisture⇌dry.