Instability of bacterial cloning vectors can present a serious problem when direct selection for plasmid-encoded phenotypes is undesirable, ineffective or impractical. Antibiotic selection may provide a satisfactory solution in enclosed fermentors but not where recombinant organisms are part of complex microbial consortia after release into the outside environment. In the past decade there has been significant progress towards understanding the causes of plasmid loss and the lessons learned from these studies can be used in the design of a new generation of stable vectors.