The positive-sense transcripts of Sindbis virus (SINV) resemble cellular mRNAs in that they possess a 5′ cap and a 3′ poly(A) tail. It is likely, therefore, that SINV RNAs must successfully overcome the cytoplasmic mRNA decay machinery of the cell in order to establish an efficient, productive infection. In this study, we have taken advantage of a temperature-sensitive polymerase to shut off viral transcription, and we demonstrate that SINV RNAs are subject to decay during a viral infection in both C6/36 (Aedes albopictus) and baby hamster kidney cells. Interestingly, in contrast to most cellular mRNAs, the decay of SINV RNAs was not initiated by poly(A) tail shortening in either cell line except when most of the 3′ untranslated region (UTR) was deleted from the virus. This block in deadenylation of viral transcripts was recapitulated in vitro using C6/36 mosquito cell cytoplasmic extracts. Two distinct regions of the 319-base SINV 3′ UTR, the repeat sequence elements and a U-rich domain, were shown to be responsible for mediating the repression of deadenylation of viral mRNAs. Through competition studies performed in parallel with UV cross-linking and functional assays, mosquito cell factors - including a 38-kDa protein - were implicated in the repression of deadenylation mediated by the SINV 3′ UTR. This same 38-kDa protein was also implicated in mediating the repression of deadenylation by the 3′ UTR of another alphavirus, Venezuelan equine encephalitis virus. In summary, these data provide clear evidence that SINV transcripts do indeed interface with the cellular mRNA decay machinery during an infection and that the virus has evolved a way to avoid the major deadenylation-dependent pathway of mRNA decay.