We have designed a method for inserting foreign DNA segments into bacteriophage T4. A plasmid containing T4 DNA is opened within the T4 sequence and the foreign DNA is inserted in vitro. Recombination in vivo, between T4 and the doubly chimeric plasmid, results in insertion of the foreign DNA into the genome of viable T4 phage. We have demonstrated the method by inserting a 203-bp DNA fragment from the lactose operon of Escherichia coli, into the dispensable region of the rIIB gene of T4. With minor modifications, the method should make possible the cloning of very large DNAs into any one of a large number of sites on the T4 chromosome.