An important goal in protein engineering is to control associations between designed proteins. This is most often done by fusing known, naturally occurring oligomerization modules, such as leucine zippers   , to the proteins of interest   . It is of considerable interest to design or discover new oligomerization domains that have novel binding specificities      in order to expand the 'toolbox' of the protein engineer and also to eliminate associations of the designed proteins with endogenous factors. We report here a simple genetic selection scheme through which to search libraries for peptides that are able to mediate homodimerization or higher-order self-oligomerization of a protein in vivo. We found several peptides that support oligomerization of the lambda repressor DNA-binding domain in Escherichia coli cells, some of them as efficiently as the endogenous dimerization domain or the GCN4 leucine zipper. Many are very small, comprising as few as six residues. This study strongly supports the notion that peptide sequence space is rich in small peptides, which might be useful in protein engineering and other applications.