The results of the experiments in killing effect of formaldehyde solution on Moina irrasa and Zoothamnium sp. reveal that the relationship between the survial ration of M. irrasa and concen tration of formaldehyde solution can be expressed by the following equations:S₁=-1.3770P+102.3333 {1-7 ≤ p ≤ 21.7}Treating for 12 hrs lnS₁=-12.2331 lnp+41.9094 21. S₂=-l.88lOp+102.2143 {1.2 ≤ p ≤ 20.8}Treating for 24 hrs lnS₂=-12.2081 lnp+41.1970 {20.8 ≤ p ≤ 29.0} All the functions between the lethal ratio Zoothamnium sp. and concentration of formaldehyde solution are as follows:R₁=4.2049p-13.5045(treating for 12 hrs) R₂=5.4960p-7.0833(treating for 24 hrs) in the paper, two effective measures are given to control the infection of Zoothamnium sp.:(1) By using 13 mg/L formaldehyde solution (treating for 12 hrs), it can kill almost all Zootham-nium sp. and, at the same time, keep the survival ratio of the water fleas at the level of 66%; (2) By using 6.5-17. lmg/L formaldehyde solution (treating for 24 hrs), it can kill 29%-87% of Zoothamnium sp. and keep the survival ratios of the water fleas correspondingly at 90%-70%.