Suppressor-sensitive (sus) mutants of bacteriophage phi80 defective in late functions were classified, by means of in vitro assembly tests, into two complementation groups: head donors and tail donors. Each group of mutants was subdivided, by means of two-factor crosses, into six cistrons. Deletion mapping revealed clustering of tail and also of head cistrons. The two clusters were located in the left arm of vegetative phi80 (the tail specifying cluster being distal). In vitro cross complementation between phi80 and lambda sus mutants revealed that whereas lambda heads could quite efficiently bind phi80 tails to form viable phage, the union of phi80 heads and lambda tails was very much less efficient. Deletion mapping of the phi80 sus mutants, using both phi80 and i(phi80)h(lambda) deletion lysogens indicated congruent gross gene arrangement in the two related bacteriophages.