We examined the effects of the anesthetic agcnt MS-222 on glochidial viability in Anodonta japonica, the parasitization rate of thc glochidia on thc host fish Gymnogobius urotaenia and the metamorphosis rate into juveniles in vitro. Snap frequency of the glochidial valves decreased with an incrcase in MS-222 concentration. Likewise, parasitization success of the larvae on thc gills of G. urotaenia was reduced when thc host fish was exposed to an MS- 222 solution (150 mg^-1L) for five minutes prior to introducing the glochidia to the fish. Metamorphosis succcss (juveniles/free-living larvae ratio) was reduced by 50% when the host fish was anesthetized with MS-222, though thc effect of the anesthetic agent was not detectable statistically. These suggest that glochidial viability and parasitization rate data obtained from previous studies using MS-222 may have contained an artifact.Therefore, future studies on the parasite-host relationship between unionid glochidia and host flshes when MS-222 is used should consider the effect of the anesthetic agent on unionid glochidia.