Human astroviruses are nonenveloped, positive-sense single-strand RNA viruses associated with self-limiting diarrhea. Although they are recognized as a leading cause of disease in young children, the cellular factors involved in astrovirus replication are not well defined. The extracellular signal-regulated kinase (ERK) pathway has been shown to regulate many viral infections, but its role during astrovirus infection is unknown. In this report, we show that astrovirus activates ERK1/2 early in infection independently of replication. Inhibition of ERK activation with U0126, a specific ERK inhibitor, significantly reduced viral production. Investigations into the mechanism of ERK1/2 regulation revealed that all steps of the viral life cycle, including early and late protein expression as well as subgenomic and genomic RNA transcription, were diminished during U0126 treatment of monolayers. These data support a role for ERK1/2 in a postattachment step, although the precise mechanism remains under investigation.