The role of the coat protein of potato virus X (PVX) was investigated by site-directed mutation of the coat protein gene. Mutant viruses with in-frame deletions in the 5' end of the coat protein gene were capable of systemically infecting plants, but produced virions with atypical morphology. Viruses with a frameshift mutation near the 5' end or with deletions in the central part of the coat protein gene failed to accumulate at detectable levels, even in the inoculated leaf. In protoplasts, mutants that infected systemically either had a wild-type phenotype or showed a small reduction in accumulation of genomic RNA. The other mutants, which did not accumulate in the inoculated leaf, were unaffected in genomic RNA accumulation 8 hr postinoculation, but at 16 hr and later they accumulated less genomic RNA than wild-type virus. None of the mutations had an effect on accumulation of negative-strand RNA. The data indicate that efficient accumulation and spread of PVX, even in the inoculated leaf, requires coat protein production and encapsidation of the viral RNA.