Stocks of bacteriophage M13 are usually grown in liquid culture. The infected bacteria do not lyse but, instead, grow at a slower than normal rate to form a dilute suspension. The inoculum of bacteriophage is almost always a freshly picked plaque or a suspension of bacteriophage particles obtained from a single plaque, as described here. Infected cells contain up to 200 copies of double-stranded, replicative-form DNA and extrude several hundred bacteriophage particles per generation. Thus, a 1-mL culture of infected cells can produce enough double-stranded viral DNA (1–2 mg) for restriction mapping and recovery of cloned DNA inserts and sufficient single-stranded DNA (∼5–10 mg) for site-directed mutagenesis, DNA sequencing, or synthesis of radiolabeled probes. The titer of bacteriophages in the supernatant from infected cells is so high (∼1012 pfu/mL) that a small aliquot serves as a permanent stock of the starting plaque.