Daphnids inhabit a diverse array of aquatic environments and they are a good model for understanding response and adaptation to environmental changes and they have been used one of standard organisms in ecotoxicology. Recent progress of genomics changed the tools for analyzing responses of daphnids, because gene expression changes can be observed before the emergence of prominent adverse effect such as immobility of the organism. Thus understanding of biological changes from gene expression level can be one of the sensitive tools for the evaluation of environmental response of organisms. However, there was no technique for genetic manipulation in daphnids. Hence, we have developed a gene introduction technique based on electroporation. There are two critical points for the successful introduction of foreign DNA into D. magna. (1) Injection of DNA into blood stream. (2) Usage of very low voltage for the electroporation. The injected DNA containing green fluorescent protein (GFP) could be introduced daphnids and the expression of GFP could be detected in living daphnids. This is the first report of gene introduction to daphnids and, together with the emerging genome sequences, will be useful for the expanding our use of daphnid in ecotoxicology.