Synopsis Introduction. Historical review. Material and methods. Checklist of flagellate symbiotes. Host-symbiote list. Systematic account. 1. Order Oxymonadida. 2. Family Oxymonadidae. 3. Genus Oxymonas Janicki. 4. Genus microrhopalodina Grassi and Foa. 5. Family Pyrsonymphidae. 6. Genus Dinenympha Leidy. 7. Genus Pyrsonympha Leidy. 8. Order Trichomonadida. 9. Family monocercomonadidae. 10. Genus Hexamastix Alexeieff. 11. Genus Tricercomitus Kirby. 12. Family Devescovinidae. 13. Subfamily Devescovininae. 14. Genus Devescovininae. 15. Genus Devescovina Foa. 16. Genus Caduceia Franca. 17. Genus Foaina Janicki. 18. Genus Polymastigoides Grasse and Hollande. 19. Subfamily Gigantomonadinae. 20. Genus Macrotrichomonas grassi. 21. Family Calonymphidae. 22. Genus Stephanonynpha Janicki. 23. Family Trichomonadidae. 24. Subfamily Trichomonadinae. 25. Genus Trichomonas Donne. 26. Subfamily Trichomitopsiinae. 27. Genus Trichomitopsis Kofoid and Swezy. 28. Order Hypermastigida. 29. Suborder Lophomonadina. 30. Family Joeniidae. 31. Genus Joenopsis Cutler. 32. Genus Parajoenopsis Saleem. 33. Suborder Spirotrichonymphina. 34. Family Holomastigotidae. 35. Genus Holomastigotoides Grassi and Foa. 36. Family Spirotrichonymphdae. 37. Genus Microjoenia Grassi. 38. Genus Spirotrichonympha Grassi and Foa. 39. Genus Holomastigotes Grassi. 40. Suborder Trichonymphina. 41. Family Eucomonymphidae. 42. Genus Pseudotrichonympha Grassi and Foa. 43. Family Teratonymphidae. 44. Genus Teranympha Koidzumi. 45. Family Trichonymphidae. 46. Genus Protrichonympha Saleem. 47. Genus Trichonympha Leidy. Distribution and Host correlation. Coevolution. Acknowledgement. Summary. References.