To the Editor.— In reading the article by Drs Weese and Smith titled "A Study of 57 Cases of Actinomycosis Over a 36-Year Period," which appeared in the December 1975 issue of theArchives(135:1562-1568, 1975), I find it dismaying that the authors persist in calling Actinomyces a fungus. In the late 1800s, it was thought to be a fungus by the original describers, but since 1957, it has been believed to fulfill all the criteria of a bacterium by the authors of major microbiological texts. True, it is filamentous and clinically behaves like a fungal infection, but it is classified as a bacterium for the following reasons: (1) it is prokaryotic (no nuclear membrane); (2) it has a filament diameter of 1μ, much smaller than the smallest fungal filament; (3) it has a cell wall of muramic acid and diaminopimelic acid like the bacteria and a lack of chitin