Actinomycetes are gram-positive bacteria. Like other bacteria they are prokaryotes and have not eukaryotic nuclei. Their ribosomes are similar to bacterial ribosomes but not to eukaryotic ribosomes. Cell walls of actinomycetes contain muramic acid like in other bacteria. In the same time the morphology of actinomycetes has some similarities to fungal morphology. So this group of bacteria were thought to be fungi for many years before more precise methods of classification became available. These prokaryotes have filamentous forms and some species form aerial mycelia in culture like fungi. Actinomycetes also form sporangia or conidia during their reproduction like some microscopic fungi.