There are a number of methods available for screening either potential recombinant clones or natural isolates for the possession of plasmids. The method described in this chapter is based on that described by Sherratt (1 ) and is the simplest and most rapid of these. The name arises from the fact that the technique was devised to yield results using a single colony that can be taken directly from an appropriate agar plate. Although this procedure works best for high copy number plasmids, it can also be used to detect plasmids in environmental isolates, for example.