One hundred and thirty actinomycetes representing 19 genera and 50 species were compared in a numerical phenetic survey using 108 unit characters. Data were examined using the simple matching (SSM), Jaccard (SJ) and pattern (DP) coefficients and clustering was achieved using both the single and unweighted pair group average algorithms. Cluster composition was barely affected by the statistics used or by test error, estimated at 2.1%. Over 80% of the strains were assigned to 2 clusters containing between two and 25 organisms. Most of the clusters were distinct and homogeneous though two were divided into subclusters. Some of the clusters and subclusters were equated with the established taxa Actinomadura madurae, Actinomadura pelletieri, Dermatophilus congolensis, Geodermatophilus obscurus, Microbispora spp., Micromonospora spp., Micropolyspora brevicatena, Micropolyspora faeni, Nocardia spp., Nocardiopsis (Actinomadura) dassonvillei, Planobispora spp., Planomonospora spp., Saccharomonospora viridis, Streptomyces somaliensis, Thermoactinomyces candidus, Thermoactinomyces dichotomica, Thermoactinomyces sacchari, Thermoactinomyces vulgaris and "Thermomonospora fusca'. The numerical data, together with results from previous chemical and genetical studies, provide sufficient evidence for the transfer of Micropolyspora brevicatena to Nocardia as Nocardia brevicatena comb. nov.