Feline calicivirus (FCV) nonstructural proteins are translated as part of a large polyprotein that undergoes autocatalytic processing by the virus-encoded 3C-like proteinase. In this study, we mapped three new cleavage sites (E46/A47, E331/D332, and E685/N686) recognized by the virus proteinase in the N-terminal part of the open reading frame 1 (ORF1) polyprotein to complete the processing map. Taken together with two sites we identified previously (E960/A961 and E1071/S1072), the FCV ORF1 polyprotein contains five cleavage sites that define the borders of six proteins with calculated molecular masses of 5.6, 32, 38.9, 30.1, 12.7, and 75.7 kDa, which we designated p5.6, p32, p39 (NTPase), p30, p13 (VPg), and p76 (Pro-Pol), respectively. Mutagenesis of the E to A in each of these cleavage sites in an infectious FCV cDNA clone was lethal for the virus, indicating that these cleavages are essential in a productive virus infection. Mutagenesis of two cleavage sites (E1345/T1346 and E1419/G1420) within the 75.7-kDa Pro-Pol protein previously mapped in bacterial expression studies was not lethal.