The scanning electron microscopy（SEM）has already been exploited in the medical and biological fields of research. Since SEM observation requires to place specimen in a vacuum chamber, it is impossible to observe the wet biological specimens like planktons as they are. Furthermore, it is required for the insulating specimens to be subjected to pretreatments such as coating of metal and carbon by vapor deposition or sputtering in order to prevent from charging the specimens. Thus, the pretreatment of the biological specimens is required for SEM observation. The conventional method generally consists of fixation, dehydration, freeze−drying, and Au sputtering. However, this method takes a lot of times and steps. Recently it is reported that the SEM observation of biological specimens can be simple by the pretreatment with room temperature ionic liquid（RTIL）. RTILs are liquid salts even at or below room temperature, and have several attractive features such as negligible vapor pressure and high ionic conductivity. The pretreatment with RTIL consists of immersing the specimens in the RTIL solution or spraying the RTIL solution onto the specimens, and subsequently blowing off the excess ionic liquids. In the present study, the pretreatment with RTIL is applied to the SEM observation of planktons as a typical sample of the wet biological species, and the optimum protocol is investigated. Ionic liquid of 1－ Ethyl－3－methylimidazolium methylphosphonate（EMI−BF4 : KantoChemical Co., Inc.,）is used in the experiment. This ionic liquid is diluted by ethanol. The clear SEM image of Artemia saline as a plankton is successfully obtained by using the pretreatment with the ionic liquid. It is found that there is the optimum concentration of the ionic liquid［1％（w/w）］for the SEM observation of planktons.