The classification of eukaryotic microorganisms, commonly referred to as protists, has been in flux for over two centuries. Recent studies have led to the proposal of multiple kingdoms rather than a single kingdom. The kingdom Protozoa, originally a class, was elevated to a kingdom by Owen in the 19th century. However, due to difficulties in distinguishing Protozoa from Animalia and Plantae, new criteria based on electron microscopy and molecular data have been used to refine the classification. The establishment of the kingdom Chromista (brown algae and diatoms) and Archezoa (primitively amitochondrial eukaryotes) has excluded many phagotrophic and aerobic protists from the Protozoa kingdom. The author argues for a more conservative approach, including all eukaryotes except Archezoa and the four major eukaryotic kingdoms (Animalia, Fungi, Plantae, and Chromista) in the Protozoa kingdom, divided into subkingdoms, infrakingdoms, parvkingdoms, and superphyla. The kingdom Protozoa includes all eukaryotes except Archezoa and the four major eukaryotic kingdoms. The author discusses the distinction between Protozoa and other kingdoms, including Plantae, Fungi, Chromista, Animalia, and Archezoa. The distinction between Protozoa and Plantae is based on the presence of plastids with double envelopes, starch in plastids or cytosol, and the absence of phagotrophy. The distinction between Protozoa and Fungi is based on the presence of chitinous walls, plate-like cristae, and the absence of phagotrophy. The distinction between Protozoa and Chromista is based on the location of chloroplasts in the RER and the presence of a unique periplastid membrane. The distinction between Protozoa and Animalia is based on the presence of collagenous connective tissue. The distinction between Protozoa and Archezoa is based on the absence of mitochondria, peroxisomes, and dictyosomes. The author proposes a revised classification of the Protozoa kingdom down to the level of subclass, including new parvkingdoms and phyla. The classification includes the kingdom Protozoa divided into two subkingdoms, two branches, two infrakingdoms, and seven parvkingdoms. The author also discusses the inclusion of Mesozoa and Myxozoa in the Protozoa kingdom. The classification includes new parvkingdoms such as Alveolata, Actinopoda, Neosarcodina, and Ciliomyxa. The author concludes that the kingdom Protozoa is a valid and well-defined taxon that includes all eukaryotes except Archezoa and the four major eukaryotic kingdomsThe classification of eukaryotic microorganisms, commonly referred to as protists, has been in flux for over two centuries. Recent studies have led to the proposal of multiple kingdoms rather than a single kingdom. The kingdom Protozoa, originally a class, was elevated to a kingdom by Owen in the 19th century. However, due to difficulties in distinguishing Protozoa from Animalia and Plantae, new criteria based on electron microscopy and molecular data have been used to refine the classification. The establishment of the kingdom Chromista (brown algae and diatoms) and Archezoa (primitively amitochondrial eukaryotes) has excluded many phagotrophic and aerobic protists from the Protozoa kingdom. The author argues for a more conservative approach, including all eukaryotes except Archezoa and the four major eukaryotic kingdoms (Animalia, Fungi, Plantae, and Chromista) in the Protozoa kingdom, divided into subkingdoms, infrakingdoms, parvkingdoms, and superphyla. The kingdom Protozoa includes all eukaryotes except Archezoa and the four major eukaryotic kingdoms. The author discusses the distinction between Protozoa and other kingdoms, including Plantae, Fungi, Chromista, Animalia, and Archezoa. The distinction between Protozoa and Plantae is based on the presence of plastids with double envelopes, starch in plastids or cytosol, and the absence of phagotrophy. The distinction between Protozoa and Fungi is based on the presence of chitinous walls, plate-like cristae, and the absence of phagotrophy. The distinction between Protozoa and Chromista is based on the location of chloroplasts in the RER and the presence of a unique periplastid membrane. The distinction between Protozoa and Animalia is based on the presence of collagenous connective tissue. The distinction between Protozoa and Archezoa is based on the absence of mitochondria, peroxisomes, and dictyosomes. The author proposes a revised classification of the Protozoa kingdom down to the level of subclass, including new parvkingdoms and phyla. The classification includes the kingdom Protozoa divided into two subkingdoms, two branches, two infrakingdoms, and seven parvkingdoms. The author also discusses the inclusion of Mesozoa and Myxozoa in the Protozoa kingdom. The classification includes new parvkingdoms such as Alveolata, Actinopoda, Neosarcodina, and Ciliomyxa. The author concludes that the kingdom Protozoa is a valid and well-defined taxon that includes all eukaryotes except Archezoa and the four major eukaryotic kingdoms