Ten temperature-sensitive mutants of Sendai virus, a paramyxovirus, were isolated and partially characterized. The mutants replicated in chicken embryo lung cells at 30 C, but not at 38 C; wild-type virus grew equally well at both temperatures. Complementation tests divided the mutants into seven groups. Six groups synthesized neither infectious virus nor RNA when incubated at 38 C from the beginning of infection. Temperature shift-up experiments demonstrated that three of these complementation groups were blocked in early steps required for RNA synthesis, but these gene functions were not needed throughout the replicative cycle. In contrast, the other three RNA-negative complementation groups were defective throughout the replicative cycle in functions required for virus-specific RNA synthesis. Only one mutant, which complemented all of the above, synthesized RNA but not infectious virus when placed at 38 C; the hemagglutinin of this mutant functioned only at the permissive temperature.