The total number of platable bacteria in the wet mud material characterized by the predominant occurrence of high-magnesian calcite and protodolomite of the Lake Balaton fluctuates between 3.5 X 10(4) and 2.3 X 10(5) g-1. Actinomycetes constitute about 25-30 per cent of this mud-community and not more than half of them belong to the genus Micromonospora. The presence of 10 species of this genus was detected among the isolated and studied strains identified by means of conventional methods and computer aided numerical analysis in which 20 authentic, mostly type strains of Micromonospora spp. were also involved. All of the three species of gentamicin-producing Micromonospora (M. echinospora, M. purpurea and M. sagamiensis) known so far were found in mud samples. These three species form stable but less numerous populations (less than 0.5 per cent of the total platable microflora) in the Balaton's mud, the members of which are capable to produce gentamicins at about 40-50 micrograms ml-1 level at most.