The nucleocapsid protein (NP) of Newcastle disease virus expressed in E. coli assembled as ring- and herringbone-like particles. In order to identify the contiguous NP sequence essential for assembly of these particles, 11 N- or C-terminally deleted NP mutants were constructed and their ability to self-assemble was tested. The results indicate that a large part of the NP N-terminal end, encompassing amino acids 1 to 375, is required for proper folding to form a herringbone-like structure. In contrast, the C-terminal end covering amino acids 376 to 489 was dispensable for the formation of herringbone-like particles. A region located between amino acids 375 to 439 may play a role in regulating the length of the herringbone-like particles. Mutants with amino acid deletions further from the C-terminal end (84, 98, 109 and 114 amino acids) tended to form longer particles compared to mutants with shorter deletions (25 and 49 amino acids).