The origin of DNA replication of bacteriophage f1 functions as a signal, not only for initiation of viral strand synthesis, but also for its termination. Viral (plus) strand synthesis initiates and terminates at a specific site (plus origin) that is recognized and nicked by the viral gene II protein. Mutational analysis of the 5' side (upstream) of the origin of plus strand replication of phage f1 led us to postulate the existence of a set of overlapping functional domains. These included ones for strand nicking, and initiation and termination of DNA synthesis. Mutational analysis of the 3' side (downstream) of the origin has verified the existence of these domains and determined their extent. The results indicate that the f1 "functional origin" can be divided into two domains: (1) a "core region", about 40 nucleotides long, that is absolutely required for plus strand synthesis and contains three distinct but partially overlapping signals, (a) the gene II protein recognition sequence, which is necessary both for plus strand initiation and termination, (b) the termination signal, which extends for eight more nucleotides on the 5' side of the gene II protein recognition sequence, (c) the initiation signal that extends for about ten more nucleotides on the 3' side of the gene II protein recognition sequence; (2) a "secondary region", 100 nucleotides long, required exclusively for plus strand initiation. Disruption of the secondary region does not completely abolish the functionality of the f1 origin but does drastically reduce it (1% residual biological activity). We discuss a possible explanation of the fact that this region can be interrupted (e.g. f1, M13 cloning vectors) by large insertions of foreign DNA without significantly affecting replication.