Karl Gottlieb Grell (28 December 1912, Burg an der Wupper – 4 October 1994) was a German zoologist and protistologist, known for his work on Trichoplax.

Karl Grell received his doctorate (Promotion) in 1934 from the University of Bonn, for a dissertation on the digestive tract of the common scorpionfly (Panorpa communis). Subsequently, he worked primarily on unicellular eukaryotes and the metazoan Placozoa. During the World War II, he was assigned to an anti-malarial unit in southeast Europe.

At the University of Tübingen, Grell was a professor of zoology, teaching protozoology and genetics. He led excursions to study marine protozoa.

Grell authored a German-language textbook on protozoology, Protozoologie (1st edition 1956, 2nd ed. 1968), published in English as Protozoology in 1973. From 1959 to 1983 he was a co-editor for the Archiv for Protistenkunde. In addition to his work on Trichoplax, he was known for his research on life cycles of the Foraminifera.

He was elected as an honorary member of the German Society for Protozoology in 1982, and was an Honorary President of the IX International Congress of Protozoology in Berlin in 1993.

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University of Bonn alumni
Academic staff of the University of Tübingen
Max Planck Society people
20th-century German zoologists
Protistologists
1912 births
1994 deaths