Bacteria have evolved diverse anti-phage defense mechanisms to protect against phage infection. Prophages, which are phages integrated into bacterial chromosomes, also encode defenses that protect their host. This study identifies a new type of anti-phage defense, called Tab, which blocks phage tail assembly, preventing the formation of infectious virions. Tab is constitutively expressed from a Pseudomonas aeruginosa prophage and allows the invading phage replication cycle to proceed, but blocks tail assembly. While the infected cell dies due to phage lysis proteins, no infectious phage progeny are released, protecting the bacterial community. Prophages expressing Tab are not inhibited during their own lytic cycle because they express a counter-defence protein that interferes with Tab function. Tab mediates defense by inhibiting phage tail assembly. The protein recognizes the tape measure protein produced during the replication cycle of invading phages and inhibits tail assembly, resulting in the release of defective phage intermediates lacking tails that are unable to infect other cells. Phages that encode Tab also possess a counter-defence protein that acts during their own lytic cycle to prevent self-targeting. This work characterizes a defense mechanism that directly targets and inhibits the virion assembly pathway, providing a new archetype of anti-phage defense. Tab activity targets the phage tail tape measure protein. The study identified that the TMP protein is the target of Tab activity. By replacing the TMP gene in phage DMS3 with that from phage JBD16C, which is not blocked by Tab, the phage was able to replicate in the presence of Tab, indicating that the TMP sequence is targeted by Tab. The study also found that anti-Tab, a protein encoded by the JBD26 prophage, binds to and inactivates Tab, allowing phages to escape Tab-mediated inhibition of replication. Tab defence systems include anti-Tab proteins that inhibit activity during the lytic cycle. The study found that phage JBD26 is resistant to Tab activity despite having a TMP sequence similar to targeted phages. This is because JBD26 encodes an anti-Tab protein that binds to and inactivates Tab, allowing JBD26 replication to proceed normally. The study also found that Tab-mediated inhibition is linked to a sequence in the C-terminal region of the TMP. Tab binds to this region and either prevents the tail assembly chaperone from binding or blocks interaction of the TMP with the baseplate. Tab-mediated inhibition of virion assembly provides a new type of anti-phage defence. While previously identified immune systems have been shown to detect phage virion components, they have not been shown to inhibit viral assembly. Instead, they act through an active abortive infection mechanism controlled by the cell. Tab-mediated inhibition of virion assembly is a new feature. The study also found that Tab defence is reminiscent of the interference tactics employed by phageBacteria have evolved diverse anti-phage defense mechanisms to protect against phage infection. Prophages, which are phages integrated into bacterial chromosomes, also encode defenses that protect their host. This study identifies a new type of anti-phage defense, called Tab, which blocks phage tail assembly, preventing the formation of infectious virions. Tab is constitutively expressed from a Pseudomonas aeruginosa prophage and allows the invading phage replication cycle to proceed, but blocks tail assembly. While the infected cell dies due to phage lysis proteins, no infectious phage progeny are released, protecting the bacterial community. Prophages expressing Tab are not inhibited during their own lytic cycle because they express a counter-defence protein that interferes with Tab function. Tab mediates defense by inhibiting phage tail assembly. The protein recognizes the tape measure protein produced during the replication cycle of invading phages and inhibits tail assembly, resulting in the release of defective phage intermediates lacking tails that are unable to infect other cells. Phages that encode Tab also possess a counter-defence protein that acts during their own lytic cycle to prevent self-targeting. This work characterizes a defense mechanism that directly targets and inhibits the virion assembly pathway, providing a new archetype of anti-phage defense. Tab activity targets the phage tail tape measure protein. The study identified that the TMP protein is the target of Tab activity. By replacing the TMP gene in phage DMS3 with that from phage JBD16C, which is not blocked by Tab, the phage was able to replicate in the presence of Tab, indicating that the TMP sequence is targeted by Tab. The study also found that anti-Tab, a protein encoded by the JBD26 prophage, binds to and inactivates Tab, allowing phages to escape Tab-mediated inhibition of replication. Tab defence systems include anti-Tab proteins that inhibit activity during the lytic cycle. The study found that phage JBD26 is resistant to Tab activity despite having a TMP sequence similar to targeted phages. This is because JBD26 encodes an anti-Tab protein that binds to and inactivates Tab, allowing JBD26 replication to proceed normally. The study also found that Tab-mediated inhibition is linked to a sequence in the C-terminal region of the TMP. Tab binds to this region and either prevents the tail assembly chaperone from binding or blocks interaction of the TMP with the baseplate. Tab-mediated inhibition of virion assembly provides a new type of anti-phage defence. While previously identified immune systems have been shown to detect phage virion components, they have not been shown to inhibit viral assembly. Instead, they act through an active abortive infection mechanism controlled by the cell. Tab-mediated inhibition of virion assembly is a new feature. The study also found that Tab defence is reminiscent of the interference tactics employed by phage