RNA 2 of beet necrotic yellow vein virus carries the cistron for the 21 kd coat protein at its 5'-extremity. During translation, the coat protein cistron termination codon is suppressed about 10% of the time so that translation continues into the adjacent open reading frame to produce a 75 kd species, known as P75, which contains the coat protein sequence at its N-terminus. Immunoblotting experiments with a P75-specific antiserum showed that P75 is present in only trace amounts in purified virus preparations. Electron microscopic visualization of immunogold-labelled virions in crude tissue extracts has provided evidence for an association between P75 and at least a fraction of the BNYVV particles, with P75 being predominantly located near one end of the rod-shaped virions. This finding is discussed in the context of the current model for the role of P75 in virus assembly and vector transmission.