Some new species of Mastigophora and Sporozoa and aMicrofilaria, parasitic in fish, frogs and toads in eastern Canada, are described and illustrated.Trypanosoma percaen.var.canadensisfrom the yellow perch,Perca flavescens, is monomorphic, smaller, with a shorter, broader body and shorter free flagellum thanTrypanosoma percaefrom EuropeanPerca fluviatilis.Trypanosoma myoxocephalin.sp. from the longhorn sculpin,Myoxocephalus octodecimspinosus, is small, with a narrow undulating membrane. It is the first trypanosome to be recorded fromMyoxocephalus.Trypanosoma lavalian.sp. fromBufo americanusis delicate, with a prominent undulating membrane with two myonemes.Trypanosoma gaumontisn.sp. fromBufo americanusis small, crook-like, with undulating membrane with deep folds and one myoneme very-near the bordering flagellum. There is no free flagellum.Trypanosoma montrealisn.sp. fromBufo americanusis long and sinuous, with very short free flagellum and without chromatoid granules in the cytoplasm.Trypanosoma rotatorium(Mayer) is described fromRana catesbianafrom eight sites,R. clamitansfrom three sites, andR. pipiensfrom three sites in Quebec Province.Trypanosoma inopinatumEd. & Et. Sergent is recorded fromRana catesbianaandR. pipiens, A form similar in dimensions and morphology except for a much shorter free flagellum has been found inR. catesbianafrom St Eustache.Spirochaeta manitouin.sp. is described from the blood ofRana catesbianafrom Lake Manitou.Lankesterella canadensisn.sp. fromRana catesbianafrom Montreal and Lake Manitou is described and illustrated. Intracorpuscular forms have polar vacuoles, schizogony occurs in endothelial cells of the blood vessels of the liver and kidney, and multinucleate oocysts have been observed.Haemogregarina myoxocephalin.sp. from the longhorn sculpin,Myoxocephalus octo-decimspinosus, andHaemogregarina urophysisn.sp. from the white hake,Urophysis tenuis, are described. Two other haemogregarines sens lat. from species ofRanaandBufoamericanusare described,Haemogregarina aeglifiniHenry is recorded from a new host,Gadus callariasfrom off Labrador,Haemogregarina platessaeLebailly fromPseudopleuronectes americanusandHaemogregarina bigeminaLaveran & Mesnil from the eel pout,Zoarces angularis, the hosts being new.Plasmodium catesbianan.sp. fromRana catesbianafrom the Back River, Montreal, is a small species, with ‘signet ring’ and amoeboid trophozoites, schizonts with up to eight merozoites clustered round coarse pigment granules and gametocytes showing slight sexual dimorphism.Plasmodium bufonisn.sp. occurs inBufo americanusfrom Laval des Rapides. The schizonts are large, oval, and give rise to eight merozoites arranged en barillet, the pigment usually being at one pole. Possibly micro- and macromerozoites may be produced. Macrogametocytes are oval and deep-staining; microgametocytes are vermicular. Both have pigment.A haemosporidian, different from but probably allied toHaemogregarina bigemina, is described from the black sea bass,Centropristis striatus.Haemoproteus laurentiaen.sp. occurs inBufo americanusfrom the banks of the St Lawrence at Montreal South and at Lake Manitou. Its pigment is characteristic, consisting of rod-like or bacilliform distributed granules.Haemoproteus lavalian.sp. fromBufo americanus, from Laval des Rapides, has pigment restricted to curved bands of rounded to somewhat angular granules. Uninucleate parasites have been observed in extremely few endothelial cells of the lungs.Haemoproteus lanoraiean.sp. fromBufo americanus, from Lanoraie, has very pale yellow to light brown pigment granules, densely crowded at one pole and forming a compact granular cap. Oval parasites have been found in endothelial cells of the lungs.Leucocytozoon salvelinin.sp., parasitic in the leucocytes of the speckled trout,Salve-linus fontinalis, is described.Dactylosoma salvelinin.sp., parasitic inSalvelinus fontinalisfrom a Laurentian stream, is the second species ofDactylosomato be described and the third to be notified from fish. It has fan- to wedge-shaped schizonts producing eight merozoites. Its gametocytes are not markedly different from one another in shape but differ in cytoplasmic staining and in nuclear structure.Dactylosomasylvatica n.sp., parasitic inRana sylvaticafrom near Quebec City, produces eight merozoites and has oval and reniform gametocytes.The morphology of aMicrofilariafromRana sylvaticais described.The effects of the Protozoa on the host cells are described for each organism, and some differential leucocyte counts for comparable normal and parasitized Amphibia are given in the concluding section.