This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The purpose of this study was to develop and apply a multilocus sequence typing 27 (MLST) scheme to study the molecular epidemiology of Brachyspira hyodysenteriae, 28 the aetiological agent of swine dysentery. Sequences of seven conserved genomic loci 29 were examined in 111 B. hyodysenteriae strains. Fifty-eight of these previously had 30 been analysed by multilocus enzyme electrophoresis (MLEE), and for some the 31 results of pulsed field gel electrophoresis (PFGE), restriction endonuclease analysis 32 (REA) and/or serotyping also were available. The discriminatory power of these 33 methods was compared. The strains were divided into 67 sequence types (STs) and 46 34 amino acid types (AATs) by MLST. The Index of Association value was significantly 35 different from zero, indication that the population was clonal. Eleven clonal 36 complexes (Cc) comprising between 2 and 10 STs were recognised. A population 37 snapshot based on AATs placed 77.5% of the isolates from 30 of the AATs into one 38 major cluster. The founder type AAT9 included 13 strains from nine STs that were 39 isolated in Australia, Sweden, Germany and Belgium, including one from a mallard. 40The MLST results were generally comparable to those produced by MLEE. The 41 MLST system had a similar discriminatory power to PFGE, but was more 42 discriminatory than REA, MLEE or serotyping. MLST data provided evidence for 43 likely transmission of strains between farms, but also for the occurrence of temporal 44 "micro-evolution" of strains on individual farms. Overall, the MLST system proved to 45 be a useful new tool for investigating the molecular epidemiology and diversity of B. 46 hyodysenteriae. 47 48