Perceived inefficiency and inadequate knowledge of the human respiratory syncytial virus (hRSV) assembly process present a hurdle for large-scale production of authentic hRSV virus-like particles (VLPs) for vaccine purposes. We previously established that the matrix protein, phosphoprotein (P), and fusion protein carboxy-terminus were sufficient to generate VLPs that resemble filamentous wildtype hRSV. Here, the contribution of P was examined. By co-expressing matrix, fusion, and modified P proteins, a ser/thr-rich P region (residues 39-57) was found to be critical for VLP formation, whereas the oligomerization domain was not. Substitutions throughout region 39-57 inhibited VLP formation and relevant amino acids were identified. Phosphomimetic substitutions of serines and threonines inhibited VLP formation; Phosphoblatant substitutions did not. The data show that P not only co-regulates replication and transcription but also has an important role in assembly, mediated by a separate domain that likely interacts with M and/or F and is highly regulated by phosphorylation.,Copyright © 2019 Elsevier Inc. All rights reserved.