Under conditions of endoplasmic reticulum (ER) stress, mammalian cells induce both translational repression and the unfolded protein response that transcriptionally activates genes encoding ER-resident molecular chaperones. To date, the only known pathway for translational repression in response to ER stress has been the phosphorylation of eIF-2α by the double-stranded RNA-activated protein kinase (PKR) or the transmembrane PKR-like ER kinase (PERK). Here we report another pathway in which the ER transmembrane kinase/ribonuclease IRE1β induces translational repression through 28S ribosomal RNA cleavage in response to ER stress. The evidence suggests that both pathways are important for efficient translational repression during the ER stress response.