Parts of the thesis have been published in articles as follows: Anderson, E. S. and Smith, H. R. (1972). Fertility inhibition in strains of Salmonella typhimurium. Molec. Gen. Genet., 118, 79-84; Smith, H. R., Grindley, J. N., Grindley, N. D. F. and Anderson, E. S. (1970). Depression of F-lac in Salmonella typhimurium by a determinant for kanamycin resistance. Genet. Res. Camb., 16, 349-353; Smith, H. R., Humphreys, G. 0., Grindley, N. D. F., Grindley, J. N. and Anderson, E. S. (1973). Molecular studies of an fi+ plasmid from strains of Salmonella typhimurium. Molec. Gen. G.net., 126, 143-151; Smith, H. R., Humphreys, G. 0. and Anderson, E. S. (1974). Genetic and molecular characterisation of some non-transferring plasmids. Molec. Gen. Genet., 129, 229-242.