A total of 749 sediment and water samples were collected from 12 lakes of the Middle Plateau of Yunnan from 1983 to 1993. The diversity and biological characteristics of the aquatic actinomycetes in these lakes were studied. Sixteen genera of actinomycetes were isolated from these samples. Micromonospores assumed a notable dominance (from 39 to 89%) in the actinomycete populations of these lake sediments. Streptomycetes were the second most abundant organisms. The diversity and counts of actinomycetes varied with the season. Thermophilic actinomycetes have a wide distribution in these lakes, but their counts were smaller. The cell wall compositions of certain Micromonospora and Streptomyces strains from an alkaline lake revealed an unusual combination of glycine and isomers of diaminopimelic acid. It seems that aquatic actinomycetes play a significant role in the decomposition of organic substances, including some toxic compounds such as phenol, in these lakes. It also appears that aquatic actinomycetes are one of the important resources for screening useful enzymes and metabolites.