ABSTRACT A quantitative model is proposed for the expected degree of relationship between copies of a family of transposable elements in a finite population of hosts. Special cases of the model (in which the process of homogenization of element copies either is or is not limited by transposition rate) are presented and illustrated, using data on mobile sequences from different species. It is shown that transposition will be expected, in large populations, to result in only a rather distant relationship between transposable elements at different genomic sites. Possible inadequacies of the model are suggested and quantified.