The purification of the neck appendage protein of φ29, p12*, which is involved in the adsorption of the phage to Bacillus subtilis, is described. The purified native protein is in a dimeric form and can be assembled, in vitro, onto purified 12− particles that lack the neck appendages, suggesting that the incorporation of p12* to the rest of the phage structure is a self‐assembly process. The assembly of protein p12*in vitro follows cooperative kinetics and it occurs with an efficiency of about 4%.