The N protein of vesicular stomatitis virus, prepared in a soluble form, was found to self-assemble, and to assemble with RNAs into RNAase-resistant structures with the buoyant density of viral nucleocapsids. It selectively assembled leader RNAs over other viral transcripts. The basis for this selective encapsidation was not the relative size of the viral transcripts or the presence or absence of a 5' cap group, but was sequence-dependent. Partial-assembly experiments demonstrated that leader RNA assembly started within the first 14 nucleotides at the 5' end. Examination of known leader RNA sequences suggests that the sequence responsible for selective assembly by N protein is a five-times-repeated A residue at every third position from the 5' end of the leader chain.