Protocol describes how to construct a library of 35-45-kb fragments of genomic DNA in the double cos site cosmid vector, SuperCos-1. The steps include: Linearization and dephosphorylation of SuperCos-1 DNA; Partial digestion of high-molecular-weight DNA with MboI; Dephosphorylation of high-molecular-weight genomic DNA; Ligation of cosmid arms to genomic DNA: Packaging and plating recombinants; Isolation and analysis of recombinant cosmids: Validation of the library.