the study of protozoans (protozoan). The science had its beginnings in the latter half of the 17th century when Antonie van Leeuwenhoek (Leeuwenhoek, Antonie van) of the Netherlands first observed protozoans by means of his invention, the microscope.

Protozoans are common, and they are of particular interest to man because they cause such diseases as malaria, amoebic dysentery, and African trypanosomiasis (sleeping sickness). Certain protozoans known as foraminifera (foraminiferan), which have an extensive fossil record, are useful to geologists in locating petroleum deposits. Protozoans also serve as experimental organisms in many studies of cell and molecular biology.