We report the bacterial expression, purification, and characterization of the N-terminal domain (PNT) of the measles virus phosphoprotein. Using nuclear magnetic resonance, circular dichroism, gel filtration, and light scattering, we show that PNT is not structured in solution. We show by two complementary computational approaches that PNT belongs to the recently described class of natively unfolded proteins, further confirming its reported similarity with acidic activation domains of cellular transcription factors. We extend these results to the N-terminal domains of other Morbillivirus phosphoproteins and to the corresponding protein W of Sendai virus, a Paramyxovirus. Unstructured proteins may undergo some degree of folding upon binding to their partners, a process termed "induced folding." Using limited proteolysis in the presence of trifluoroethanol, we identified residues 27 to 38 as a putative secondary structure element of PNT arising upon induced folding.