Ten sus mutants of the staphylococcal bacteriophage phi 11, each a representative from a different complementation group, have been used in three-factor cross experiments. The results of these crosses indicate a circular genetic map for phi 11. Functional studies of the mutants have been limited to electron microscopic examinations of lysates after prophage induction (or infection). One gene is an early gene, five genes are concerned with tail formation, and three are concerned with head formation. The tenth gene is possibly a head gene. The contribution by phi 11 to the genomic content of the plasmid-phage hybrid phi 11 de has been investigated. Phi 11 de contains most of the late genes and appears to be missing a continuous phi 11 segment that includes the early gene flanked by two late genes.