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Celery latent virus (CeLV) is an incompletely described plant virus known to be sap and seed transmissible and to possess flexuous filamentous particles measuring about 900 nm in length, suggesting it as a possible member of the family Potyviridae. Here, an Italian isolate of CeLV was transmitted by sap to a number of host plants and shown to have a single-stranded and monopartite RNA genome being 11 519 nucleotides (nts) in size and possessing some unusual features. The RNA contains a large open reading frame (ORF) that is flanked by a short 5' untranslated region (UTR) of 13 nt and a 3' UTR consisting of 586 nt that is not polyadenylated. CeLV RNA shares nt sequence identity of only about 40 % with other members of the Potyviridae (potyvirids). The CeLV polyprotein is notable in that it starts with a signal peptide, has a putative P3N-PIPO ORF and shares low aa sequence identity (about 18 %) with other potyvirids. Although potential cleavage sites were not identified for the N-terminal two-thirds of the polyprotein, the latter possesses a number of sequence motifs, the identity and position of which are characteristic of other potyvirids. Attempts at constructing an infectious full-length cDNA clone of CeLV were successful following Rhizobium radiobacter infiltration of Nicotiana benthamiana and Apium graveolens. CeLV appears to have the largest genome of all known potyvirids and some unique genome features that may warrant the creation of a new genus, for which we propose the name 'celavirus'.