Measles virus encodes an RNA-dependent RNA polymerase composed of the L and P proteins. Recent studies have shown that the L proteins of both Sendai virus and parainfluenza virus 3 form an L-L complex [Cevik, B., Smallwood, S., Moyer, S.A., 2003. The oligomerization domain resides at the very Nterminus of the Sendai virus L RNA polymerase protein. Virology 313, 525-536.; Smallwood, S., Moyer, S.A., 2004. The L polymerase protein of parainfluenza virus 3 forms anoligomer and can interact with the heterologous Sendai virus L, P and C proteins. Virology 318, 439-450.; Smallwood, S., Cevik, B., Moyer, S.A., 2002. Intragenic complementation and oligomerization of the L subunit of the Sendai virus RNA polymerase. Virology 304, 235-245.]. Using differentially tagged L proteins, we show here that measles L also forms an oligomer and the L-L binding site resides in the N-terminal 408 amino acids overlapping the P binding site in the same region of L. To identify amino acids important for binding P and L, site-directed mutagenesis of the L-408 protein was performed. Seven of twelve mutants in L-408 were unable to form a complex with measles P while the remainder did bind at least some P. In contrast, all of the mutants retained the ability to form the L-L complex, so different amino acids are involved in the L and P binding sites on L. Four of the 408 mutations defective in P binding were inserted into the full-length measles L protein and all retained L-L complex formation, but did not bind P. Full-length L mutants that did not bind P were also inactive in viral RNA synthesis, showing a direct correlation between P-L complex formation and activity.