It is probably a biological constant that viruses are found associated with all known life and that they play an essential role in the ecology and evolution of all lifeforms. Lytic viruses have now been discovered that infect organisms from each of the three domains of life. Recently, a new lysis system has been described for two archaeal viruses. This lysis system appears to be novel and distinct from previously described virus-encoded lysis systems. A comparison of lytic viruses from each of the three domains may provide insights concerning the evolution of viruses.