The mini-circle is a transposable element which is present in Streptomyces coelicolor A3(2) in both free circular and chromosomally integrated linear forms. The nucleotide sequences of the mini-circle and its preferred site of integration in the Streptomyces lividans TK64 chromosome were determined. Three putative open reading frames were identified in the mini-circle sequence. The mini-circle does not appear to cause a target site duplication on transposition and does not have perfect terminal inverted repeats. The observed site-specificity of the mini-circle is not mediated by extensive homology between the element and the chromosomal integration site. Transposition of the mini-circle into the S. lividans chromosome was demonstrated and found to be some two orders of magnitude less efficient than integration of the circular form of the element, suggesting that the circular form of the mini-circle might be a normal intermediate in the transposition process.