Ribonucleic acid (RNA) species in mumps virions and in infected cells were compared. The predominant RNA species in virions labeled with (3)H-uridine sedimented at 50S; RNA species sedimenting at 28, 18, and about 10S were also present. The virion-associated RNA species sedimenting slower than 50S contained some nucleotide sequences similar to 50S virion RNA. Although mumps virus replication was severely inhibited by high concentrations of actinomycin D, some virus was made, and virus-specific RNA species accumulated in infected cells. Mumps virus resembled other paramyxoviruses in inducing, in infected cells, synthesis not only of 50S RNA but also of slower sedimenting RNA species with a peak distribution at about 18S, complementary in base sequences to 50S virion RNA. In addition, base sequences of the parental type were relatively abundant in the RNA species sedimenting slower than 50S; these may represent precursors of the slowly sedimenting RNA species associated with virions. Ribonuclease-resistant RNA was detected in infected cells; this may represent replicative or transcriptive intermediates. Inhibition of protein synthesis with cycloheximide severely depressed accumulation of labeled 50S RNA in infected cells but did not interfere with accumulation of RNA species sedimenting slower than 50S. Actinomycin D treatment had a similar effect. Annealing of genomes and virus-induced complementary RNA species of Newcastle disease virus, Sendai virus, and mumps virus did not reveal any base sequence homologies.