The morphology and ultrastructure of two strains of Borrelia anserina were investigated by electron microscopy of negatively stained and ultrathin sectioned cells. One was a cultivable strain originally isolated in the USA and the other was originally isolated in Nigeria and maintained in chickens. The cells were regularly helical, 9-21 microns long and 0.22-0.26 microns wide with a helix wavelength of about 1.7 microns. The cells were surrounded by a surface layer and appeared to divide by binary fission. The structure of the cells from each of the two strains was identical except that those of the USA strain possessed seven flagella inserted at each end and those from the Nigerian strain had eight.