Tn1000 (or ’gamma delta’) insertions in a pBR322-recombinant plasmid carrying the threonine operon (pBE) were obtained either spontaneously during chemostat cultivation (pBE-2) or selected during transconjugation (pBE-3 and pBE-4). The different insertion derivatives were tested for their stability in different host genetic backgrounds (in relation to threonine auxotrophy and recA function), various media composition (liquid or solid, rich or minimal) and growth temperatures (30 degrees C, 37 degrees C and 42 degrees C). All the derivatives carrying the gamma delta sequence, albeit increasing their size from 9.7 to 15.6 Kb, significantly enhanced segregational and structural plasmid stability in every condition tested.