In order to see whether the polyploid cells lining the mouse urinary bladder are formed by nuclear fusion, such epithelium was studied under the light and electron microscope forty-eight hours after an injection of cyclophosphamide when the bladder epithelium regenerates with rapid formation of many diploid, tetraploid and octoploid cells. The probability of seing fusion, if it occurs, ought then to be high. Serial sections of many specimens from four mice revealed no signs of fusion. Thus we found no support for the theory that polyploid cells are formed by nuclear fusion.