Two hundred and twenty-two representatives of the Actinomycetaceae and related taxa, including reference cultures, received strains and fresh isolates, were tested using 124 unit characters. The data were examined using numerical taxonomic techniques with various coefficients and average linkage clustering; the variation in composition of the clusters obtained from the different coefficients was slight. Most species included in the study formed discrete phena, which exhibited good differential characters. Actinomyces israelii was particularly well-defined and A. naeslundii and A. viscosus, although grouping together, did show divergence in several test results. Actinomyces bovis was initially linked with representatives of genera other than Actinomyces, these being Bifidobacterium, Corynebacterium pyogenes and Erysipelothrix rhusiopathiae. The strains of Bacterionema matruchotii and Rothia dentocariosa formed tight distinct clusters associated only at low similarity levels with other members of the family Actinomycetaceae. The Arachnia propionica phenon, although well-defined, appeared to consist of two subclusters which could be assigned to serotypes 1 and 2. Tests which may be useful in identification are tabulated and the relationships between the various taxa are discussed.