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 50
S
; RNA species sedimenting at 28, 18, and about 10
S
were also present. The virion-associated RNA species sedimenting slower than 50
S
contained some nucleotide sequences similar to 50
S
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 50
S
RNA but also of slower sedimenting RNA species with a peak distribution at about 18
S
, complementary in base sequences to 50
S
virion RNA. In addition, base sequences of the parental type were relatively abundant in the RNA species sedimenting slower than 50
S
; 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 50
S
RNA in infected cells but did not interfere with accumulation of RNA species sedimenting slower than 50
S
. 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.