The polycistronic P/C mRNA of Sendai virus is translated under cell-free conditions into five proteins (P, C', C, Y1, and Y2) from overlapping reading frames. In this study, we showed that in addition to the P, C', and C proteins, Y1 and Y2 were expressed by six different Sendai virus strains in infected cells. The Y proteins exhibited strain-specific variation in their gel mobility which corresponds to the variation seen in the cognate C proteins. While the relative levels of the P, C', and C proteins were consistent among various cell lines, the levels of Y1 and Y2 proteins varied among the cell lines used for viral infection.