As nouns the difference between protozoan and gregarine

is that protozoan is (cytology) any of the diverse group of eukaryotes, of the phylum protozoa, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia while gregarine is any of various sporozoan protozoans, that are parasitic in the digestive tracts of some invertebrates.

As an adjective protozoan

is of, pertaining to, or characteristic of a protozoan.