Two nonfimbriate strains of Escherichia coli and their K88(+) counterparts, obtained by episomal transfer of this antigen, were studied with the electron microscope. Specimens were prepared with the spray drop method and were studied after shadow-casting. Under these conditions, the alteration in surface structure, due to the acquisition of K88 antigen, appeared as a fur of fine filaments, distinctly more flexible than fimbriae. Purified K88 antigen was also studied and found to have the same structure.