The genome of the single-stranded DNA phage f1 contains an intergenic region (IG), 508-nucleotides long, that does not code for any known protein. By use of a system of chimeric plasmids haboring different f1 fragments, we had previously shown that this region contains, in addition to the f1 'functional origin' of DNA replication, a signal of less than 300 nucleotides required for efficient morphogenesis to occur ('morphogenetic signal'). In the present study, we have localized this signal to within a sequence of less that 60 nucleotides of almost perfect palindromic symmetry at the genet IV/IG border. We also present data indicating that the morphogenetic signal is not necessary for the synthesis of single-stranded DNA, but is necessary only at some later step during virion maturation.