Heterogeneous polyoma giant RNA molecules have been isolated by oligo(dT)- cellulose chromatography during the late phase of a lytic cycle of infection of mouse kidney cell cultures. These RNAs have sedimentation coefficients in denaturing Me2SO gradients that are greater than 26S and thus apparently correspond to RNA molecules larger than one strand of polyoma DNA. Approximately 15% of total nuclear polyoma late giant RNAs contained tracts of poly(A) and were retained by oligo(dT)-cellulose. The polyoma late giant RNAs as well as heterogeneous nuclear RNAs (HnRNAs) were found to have a slightly lower sedimentation rate in Me2SO-chloral hydrate density gradients than sedimentation values in sucrose gradients indicated. Even when synthesis of viral DNA and the production of capsid protein are blocked by 5-fluorodeoxyuridine (FdU), 10% of polyoma-specific RNA (as determined by sedimentation analyses under aqueous conditions) was shown to contain tracts of poly(A). In contrast to our findings on polyoma late giant RNA, nuclear polyoma RNA synthesized in the presence of FdU sedimented in denaturing Me2SO-chloral hydrate gradients considerably slower (from 15 to 30S) in relation to HnRNA and ribosomal precursor RNA. The sedimentation pattern in denaturing Me2SO gradients suggest that Py RNA synthesized late in lytic infection in the presence of FdU may be no longer than one transcript of Py DNA.