Abstract. Rhodococcus equi isolates (462) obtained from 64 soil samples collected on 5 R. equi-endemic horse-breeding farms and isolates from 100 infected foals in Texas were examined to determine the prevalence and genotypic diversity of virulence-associated plasmids. Isolates were tested for the presence of 15-17-kDa virulence-associated protein antigens (VapA) by immunoblotting and virulence-associated plasmids by PCR. Plasmid DNAs were isolated and analyzed by digestion with restriction endonucleases for estimation of size and comparison of polymorphisims. Rhodococcus equi were isolated from soil of all 5 farms; however, virulent R. equi were only isolated from 3 of the 5 farms and represented 18.8% (87 of 462) of total isolates. Of the 87 virulent soil isolates, 56 (64.5%) contained an 85-kb type I plasmid, 23 (26.4%) an 87-kb type I plasmid, 7 (8%) a newly defined 85-kb type III plasmid (Tx 43), and 1 (1.1%) a newly defined 85-kb type IV plasmid (Tx 47). Of the 100 isolates from infected foals, 96 were virulent. Of the 96 virulent isolates, 51 (53.1%) contained an 85-kb type I plasmid, 39 (40.6%) an 87-kb type I plasmid, 4 (4.2%) an 85-kb type III plasmid (Tx 43), and 2 (2.1%) an 85-kb type IV plasmid (Tx 47). There are at least 4 different R. equi virulenceassociated plasmids in Texas, 2 of which have not previously been described. Based upon virulence plasmid typing, there is geographic diversity among isolates of R. equi from clinical and environmental samples on horse-breeding farms in Texas. There is not a strong correlation between the presence of virulent R. equi in farm soils and the R. equi disease status of those farms.Rhodococcus equi is one of the most important bacterial pathogens of young foals. Infections caused by this organism are characterized by chronic, suppurative bronchopneumonia and enteritis associated with a high mortality rate. 1,7,18 It was previously reported that the 15-17-kDa virulence-associated protein antigens (VapA) of R. equi are associated with virulence in mice 15,19 and that the presence of an 85-or a 90-kb plasmid is essential for virulence and the expression of VapA. 10,14,[22][23][24] These virulence-associated antigens and virulence plasmids have been used as epidemiologic markers to identify virulent R. equi isolates from horses and their environment by Western blot (immunoblot) assay using a monoclonal antibody and plasmid profiles. 4,11,12,16,17,21 More recently, restriction enzyme digestion patterns of virulence plasmids in human and foal isolates from several countries were examined. 6,20 The digestion patterns divided the plasmids of virulent isolates into 5 closely related types. Three of the 5 types had already been reported in Canadian and Japanese isolates, 6 and 2 new types were identified in French and Japanese isolates. 9,20 These 5 types of plasmids were designated as 85-kb type I (p-REAT701), 85-kb type II (a new type), 87-kb type I (EcoRI and BamHI type 2), 6 87-kb type II (a new type), and 90-kb (pREL1). The 85-kb type I plasmid was found in isolat...