A tobamovirus isolated from pepper crops in Bulgaria has been characterized, and is referred to below as P101. It was closely related to Paprika mild mottle virus (PaMMV) (Dutch isolate), based upon the serological relationship of its coat protein, and the nucleotide sequence analysis of the gene encoding the coat protein and the 3' non-coding region of the viral RNA. The coat proteins of the two isolates differ by two amino acids, and these substitutions may be responsible for the different reactivity of the isolates towards a polyclonal antiserum raised against the virion of the Dutch isolate. The biological behaviour of both isolates was similar in the hosts tested, except in pepper plants where P101 induced delayed and milder symptoms compared with PaMMV, although their accumulation levels were similar. In addition, we investigated the infection pattern of the two isolates in tomato plants. Both isolates accumulated in protoplasts as well as in inoculated leaves, although systemic invasion was limited. This limited spread was not due to activation of defense mechanism(s) in the plant, since the upper uninoculated leaves from P101-infected tomato plants were fully susceptible to challenge inoculation with the virus. Instead, it appears due to a restriction of long-distance movement, that could be overcome in tomato plants co-infected with Tobacco mosaic virus (TMV), but not with either Cucumber mosaic virus or Pepino mosaic virus. The ability of P101 to move systemically in the presence of TMV was not linked to enhanced accumulation of P101 at the cellular level. Thus, a tobamovirus but not the viruses tested from other genera could complement, in trans, the function(s) required for PaMMV to invade the upper uninoculated leaves. Paprika mild mottle virus strain B is proposed as the name for this new isolate.