Abstract Terminase is a protein complex involved in λ DNA packaging. The subunits of terminase, gp Nul and gp A, are the products of genes Nul and A. The actions of terminase include DNA binding, prohead binding and DNA nicking. Phage 21 is a lambdoid phage that also makes a terminase, encoded by genes 1 and 2. The terminases of 21 and λ are not interchangeable. This specificity involves two actions of terminase; DNA binding and prohead binding. In addition, the subunits of λ terminase will not form functional multimers with the subunits of 21 terminase. λ-21 hybrid phages can be produced as a result of recombination. We describe here λ-21 hybrid phages that have hybrid terminase genes. The packaging specificities of the hybrids and the structure of their genes were compared in order to identify functional domains of terminase. The packaging specificities were determined in vivo by complementation tests and helper packaging experiments. Restriction enzyme site mapping and sequencing located the sites at which recombination occurred to produce the hybrid phages. λ-21 hybrid 51 carries the λ A gene, and a hybrid 1 Nul gene. The crossover that produced this phage occurred near the middle of the 1 and Nul genes. The amino-terminal portion of the hybrid protein is homologous to gp1 and the carboxy-terminal portion is homologous to gp Nul. It binds to 21 DNA and forms functional multimers with gp A, providing evidence that the amino-terminal portion of gp Nul is involved in DNA binding and the carboxy-terminal portion of gp Nul is involved in the interaction with gp A. λ-21 hybrid 54 has a hybrid 2 A gene. The amino terminus of the hybrid protein of λ-21 hybrid 54 is homologous with gp2. This protein forms functional multimers only with gp1, providing evidence that the amino terminus of gp A is involved in the interaction with gp Nul. These studies identify three functional domains of terminase.