The structure of mammalian orthoreovirus top component particles, which are profoundly deficient in the content of double-stranded RNA genome, was determined at 30 A resolution by transmission cryoelectron microscopy and three-dimensional image reconstruction. Previously undetected, ordered densities, appearing primarily as pentameric flowers in the reconstruction, were seen to extend 65 A inwardly from the inner capsid at the icosahedral fivefold axes. Identically positioned but lower density elements were observed in two types of partially uncoated top component particles obtained by limited proteolysis. The levels of three inner-capsid proteins-lamda 1, lamda 3, and mu 2-were reduced in concert with the internal densities during proteolytic uncoating. Since lamda 3 contains the catalytic regions of the viral RNA polymerase and since both lamda 1 and mu 2 appear to play roles in transcription or mRNA capping, the internal structures are concluded to be complexes of the viral transcriptase-related enzymes. The findings have implications for the mechanisms of transcription and mRNA capping by orthoreovirus particles.