Abstract: Recombinant parainfluenza virus (PIV) are provided in which expression of the C, D and/or V translational open reading frame(s) (ORFs) is reduced or ablated to yield novel PIV vaccine candidates. Expression of the C, D and/or V ORF(s) is reduced or ablated by modifying a recombinant PIV genome or antigenome, for example by introduction of a stop codon, by a mutation in an RNA editing site, by a mutation that alters the amino acid specified by an initiation codon, or by a frame shift mutation in the targeted ORF(s). Alternatively, the C, D and/or V ORF(s) is deleted in whole or in part to render the protein(s) encoded thereby partially or entirely non-functional or to disrupt protein expression altogether. C, D and/or V ORF(s) deletion and knock out mutants possess highly desirable phenotypic characteristics for vaccine development. These deletion and knock out mutations changes specify one or more desired phenotypic changes in the resulting virus or subviral particle.