The complete nucleotide sequence of Cocksfoot streak virus (CSV) has been determined. The viral genome comprises 9663 nucleotides, excluding a 3'-terminal poly(A) sequence. The genome of CSV has a 133 nt 5'-non coding and a 260 nt 3'-non coding region. The RNA of CSV encodes a single polyprotein of 3089 amino acid residues and has a deduced genome organization typical for a member of the family Potyviridae. CSV is transmissible by aphids and has a narrow host range in the Gramineae. It was compared to two potyviruses having monocotyledonous hosts (monocot potyviruses) and several potyviruses infecting dicotyledonous plants (dicot potyviruses). CSV is most closely related to other monocot potyviruses like Maize dwarf mosaic virus (MDMV) and Johnson grass mosaic virus (JGMV), but also closely related to the dicot potyviruses. On the other hand, CSV is less related to monocot viruses from the other genera (Rymovirus, Tritimovirus) within the Potyviridae. Specific motifs, described for potyviral polyproteins, are also present in the polyprotein of CSV. Only two motifs in the HC-Pro--motif involved in long distance movement and motif for HC-Pro self-interaction--were different in comparison to most of the dicot potyviruses.