The mechanism of penetration and uncoating of enveloped animal viruses has recently been the subject of renewed controversy. Electron microscopic studies have led a number of investigators to conclude that penetration takes place by cellular engulfment (viropexis) of intact virus particles (Dales & Choppin, 1962; Mussgay & Weibel, 1962; Dales, 1963; Simpson, Hauser & Dales, 1969). Other investigators have concluded that enveloped animal viruses penetrate by fusion of the virus envelope with the host cell plasma membrane (Morgan & Howe, 1968; Morgan, Rose & Mednis, 1968a, b; Heine & Schnaitman, 1969). We now report on the role of the Sindbis virus envelope in the attachment of virus to host cells.