The water spider, Argyroneta aquatica, is the only spider that lives under water. In Japan, due to the fragmentation and the deterioration of the habitat, a policy for conservation of this spider is needed without delay. However, there is no information about the water condition of the habitat of this species. As a study of the conservation of the water spider, we conducted field surveys on the measurement of temperature, pH, and DO (Dissolved Oxygen) of the water in Mizoro pond, which is a habitat of the water spider, in May 1996. The water in which the water spider lived had lower pH and DO than the water in which the water spider did not live. It appears that this low DO value does not allow fishes to live. Consequently, the low DO would prevent potential predators, such as fishes, to live in the habitat of the water spider, giving an enemy free space to them. This low pH and DO is thought to be kept by the activity of sphagna, which is important for habitat conservation of the water spider in Mizoro pond.