1) Temperature and water conditions of paddy fields in the cropless period after rice harvest strongly influenced the rate of multiplication of tubers of Sagittaria pygmaea.
 2) Infestation of S. pygmaea in paddy fields reduces the number of rice panicles produced, and causes a substantial yield reduction of rice.
 3) Pyrazolate was recognized as a useful herbicide against the weed, because it showed a consistent killing effect under various conditions. However, it was found that pyrazolate must be applied repeatedly for at least 2-3 years in order to get perfect effect of control.
 4) Rotary-tilling of paddy fields several times during winter was found a very effective agronomic control method against the weed.