The (+) single-stranded RNA (ssRNA) of the L-A virus is the species packaged to form new viral particles. Empty L-A viral particles specifically bind viral (+) ssRNA, and a sequence 400 bases from the 3' end is necessary for this activity. We show that its stem-loop structure, the A residue protruding from the stem, and the loop sequence are all important for the binding, and that this 34 base region is sufficient for the binding. M1, a satellite virus of L-A, has a similar structure on its (+) strand that is likewise sufficient for the binding. Heterologous RNA with the binding sequence from L-A or M1, when expressed in vivo, was packaged in L-A viral particles. Thus, the sites necessary to bind to empty particles are encapsidation signals for the L-A virus. Since the pol domain of the 180 kd minor coat protein appears to be responsible for the binding, this result suggests that the RNA polymerase molecule recognizes the viral genome for packaging.