Alfred Detlef Fritz Kahl (18 February 1877 – November, 1946) was a German schoolteacher who took up microscopy in mid-life and became a leading authority on ciliated protozoa.

In a burst of scientific productivity that lasted just nine years, he published 1800 pages of scholarly work, in which he described 17 new ciliate families, 57 genera, and about 700 previously unknown species.

His interest in ciliates began, according to his own account, when his daughter was studying under Eduard Reichenow, then head of the protozoa department at the Hamburg Tropical Institute: "The very interesting literature and preparations my daughter Lucia brought home fascinated me, as a dedicated biologist, and created the desire to study this field more deeply…Thus, I enthusiastically commenced literature reading and investigation of the small water bodies in my surroundings at the beginning of the year 1924.

Despite his simple procedures, absence of collaborators and lack of formal training, he came to dominate his field, to the extent that the period from 1930 to 1950 has been characterized as the "Kahlian era" of ciliate systematics.

This accomplishment has been described by the ciliatologist John O. Corliss as an "unbelievable record [that] has never been--and is hardly ever likely to be--met in the annals of protozoology and microscopy, past, present, or future.