Type III CRISPR–Cas adaptive immune systems are RNA-guided defence systems in archaea and bacteria. CRISPR–Cas systems use guide RNAs and Cas nucleases to bind to complementary sequences in invading nucleic acids and degrade them. Among the six different types identified so far, the type III system is the most complex. The mechanism of action of CRISPR–Cas type III-B involves target RNA cleavage via Cmr effector complexes, target RNA-activated single-stranded DNA (ssDNA) cleavage and cyclic oligoadenylate synthesis by Cas10, which in turn leads to the activation of the type III CRISPR–Cas accessory RNase Csx1. To counteract the CRISPR–Cas immunity of the host, bacteriophages and archaeal viruses have developed anti-CRISPR (Acr) proteins. To date, only a few Acr proteins have been reported to inhibit type III CRISPR–Cas immunity and the mechanism of inhibition is incompletely understood. In this new study, Lin et al. report the identification of an archaeal virus Acr protein, AcrIIIB2, that blocks the dissociation of the target RNA from the type III-B CRISPR–Cas Cmr-α effector complex, which prevents the recycling of the complex. During late stage of infection, bacterial and archaeal hosts arrest their metabolism, which enables the recycling of the effector complex and thus gradual degradation of the viral genome, a mechanism that is blocked by the newly identified Acr protein. The authors first report that the genome of Sulfolobus islandicus rod-shaped virus 3 (SIRV3; which infects its archaeal host) encodes a type III-B Acr protein, namely AcrIIIB2. Using deletion mutants, the authors went on to show that AcrIIIB2 specifically inhibits the CRISPR–Cas effector complex Cmr-α.