Neoergasilus japonicus is a parasitic copepod which has been reported from various freshwater fisher in Asia and Europe.
In this study, a morphology of the species in nauplius stages is given from the results of a resring experiment.
According to the molts, the naupliar development of the species was divided into six stages like typical free-living copepods. They are capable of feeding throughout all the stages. The first maxi-lla appesrs in stage IV, and the rudimental first and second legs in stage VI. The second maxilla and the maxilliped are never found in any of the stages.
The result was compared with the descriptions of six other species of Ergasilidae in their deve-lopmental courses of nauplius stages, and some suspicion was cast on the results of the past workers.