This paper, while partially a review of previously published reports from North America, presents many new records on the occurrence of Trichomonas gallinae in North American columbiform birds. Evidence is presented that the .infection is present throughout most of the United States and on occasion has reached epizootic proportions in mourning doves and band-tiled pigeons in isolated areas. An extensive epizootic is reported involving several thousand eastern mourning doves in the Southeastern States during the summer of 1950. Although probably the chief means of transmission is from. parent to offspring in columbiform birds (and the occurrence of infection in nestling mourning doves demonstrates that this mode of inoculation occurs) in severe epizootics there also is undoubtedly a transmission by contamination of water or food besides, or as well as, the parent to offspring transfers. When T. gallinae is implanted in clean pigeons it becomes evident that there is a variety of strains of the organisms and that they differ considerably in virulence. Some strains may kill almost every recipient, while others, though they readily infect, never produce the slightest discomfort or pathology. Nonlethal infections produce an immunity to all strains tested. In some cases, however, when these double-strain infections are further inoculated into. clean birds, the effects of either the nonlethal or the killing strain may predominate. The mechanism by which this virulence acts in the blrds is undoubtedly an important factor in determining the morbidity or mortality that may occur in a population. The prevalence of trichomoniasis in native pigeons and doves presents many practical as well as academic problems. Ultimate application of control measures, evenon an experimental basis, must await further knowledge of many more basic facts.