Polyethyleneglycol (PEG m.w. 6000) has been used to induce the fusion of unfertilized eggs of some species of Ascidians (Ciona intestinalis and Ascidia malacci). In order to obtain double giant eggs, the eggs taken from the oviducts and freed from the membranes are kept in a PEG solution side by side. For Ascidia malaca the PEG concentration optimum to obtain the fusion is of 3% or 4% for Ciona intestinalis of 6% or 10%. According to the physiological condition of the eggs the fusion can take place after 3 or 4 hours, sometimes after 12 hours. In either case the giant eggs on having completed their fusion, are isolated, put in (pasturized and filtered) sea water, and then fertilized with sperms of the same species. An almost normal development up to the larva stage only for the giant eggs of Ascidia malaca has been observed, whereas for those of Ciona intestinalis no development has been observed. This different result is probably due to the chemical agent (PEG) when it is used at a high level of concentration (6% or 10%) on the cytoplasm of the eggs. From our observations it is possible to conclude that the PEG is a good chemical agent for the fusion of eggs. This has led us to extend its use of interspecific fusion between eggs of different species these such studies are justified by the possibility of further investigations of the complex relationship of nuclear and cytoplasmic interactions and the role that the membranes would have in the mecanism of the cell fusion.