We have constructed a cosmid vector, Tropist3, based on the lambda origin double-cos-site vector Lawrist4, which is designed for efficient cloning and mapping of genomic DNA. Tropist3 contains two cloning sites in addition to the HindIII and BamHI sites present in Lawrist4; a SalI site allows cloning of Sau3AI partial digests following partial filling-in of the ends, and a PmlI site is suitable for blunt-end cloning. Both these strategies reduce the chance of co-cloning two inserts. Tropist3 also contains NotI, PacI, SacII and KpnI sites flanking the cloning region; these allow most inserts to be excised cleanly and mapped by partial digestion followed by hybridization with short vector sequences which lie adjacent to the cloning sites. This will also be useful for recloning inserts into different vectors, or for cosmid sequencing projects.