P1 is a temperate bacteriophage (phage) that infects Escherichia coli and a some other bacteria. When undergoing a lysogenic cycle the phage genome exists as a plasmid in the bacterium unlike other phages that integrate into the host DNA. P1 has an icosahedral "head" containing the DNA attached to a contractile tail with six tail fibers.

The P1 phage has gained research interest because it can be used to create the P1-derived artificial chromosome cloning vector which can carry relatively large fragments of DNA. Also, P1 encodes a site-specific recombinase, Cre, that is widely used to carry out cell-specific or time-specific DNA recombination by flanking the target DNA with loxP sites.(see Cre-Lox recombination)