A study of the organisation of bacterial chromosomes is timely. The complete nucleotide sequences of several bacterial chromosomes have recently been reported, or will become available in the near future. Understanding how these chromosomes are packaged and organised within the cell is a priority for interpreting the new sequence data and their functional implications. It has been suggested that the bacterial chromosome is divided into about 100 independently supercoiled domains although, until now, it has not been possible to study these domains in detail or to assess their functional significance. In collaboration with Professor Higgins' laboratory I have recently developed a highly sensitive and reproducible method for measuring DNA supercoiling in vivo using DNA cross-linking by trimethylpsoralen. Exploitation of this method will allow, for the first time, direct in vivo studies of the supercoiling of individual chromosomal domains, will permit the boundaries of these domains to be mapped, and the effects of transcription ('twin domain' model) and histonelike proteins on DNA supercoiling and domain structure to be assessed. These studies will lead to a much more complete understanding of the organisation and functionality of bacterial chromosomes.