A total of 105 strains of H. somnus isolated from cattle in Denmark and other countries during 1982-1951 were compared with regard to biotypes (fermentation of 8 different sugars), plasmid profiles, Taq1 restriction endonuclease analysis of chromosomal DNA (REA-typing) and EcoRI-generated DNA restriction fragment length polymorphisms of rRNA genes (ribotyping). Eighty-four strains originating from cases of pneumonia, and 21 originating from the genitals of bulls were included in this study. Biotyping yielded 21 different types. Twenty-two of the isolates contained plasmids, and these were divided into 12 distinct plasmid profiles. Analysis of chromosomal DNA restriction patterns, resulted in 33 different REA patterns and 16 different ribopatterns in the investigated strains. Biotypes, REA-types, and ribotypes generally showed good correlation, whereas plasmid profiles did not correlate with any other typing method. Seventy-eight percent of all Danish isolates from pneumonia belonged to the same REA-, and ribotype, indicating a single clone. Furthermore, a single strain from semen belonged to this type, but generally strains isolated from the genital tract showed little homology to strains isolated from cases of pneumonia with regard to REA-, and ribotypes. The typing methods applied in the present investigation appeared to be useful indicators of epidemiological relatedness between H. somnus strains from cattle, and might be used for epidemiological investigations as well as for taxonomic studies of this species