THE Moore Collection contains six hundred and nine recently taken specimens of the genus Empidonax representing every indubitable race, except E. fulvifrons fusciceps, in Mexico and Central America, as well as several forms not hitherto described. We have no representative of the unique type of Muscicapa fulvifrons Giraud, of which as Hellmayr (1927: 220) sagely remarks, the habitat remains to be discovered. In addition, about one hundred and fifty specimens collected within the past three years by other ornithologists have been made available. Supplemented by more than one thousand older skins in various museums, this enormous series has given the author an extraordinary opportunity to study this difficult genus. Five years ago he realized there was an undescribed race of E. albigularis resident in southern Sinaloa, and shortly thereafter that there were other undescribed forms in the high mountains of western Durango, Veracruz and Honduras. He did not care to separate these until an adequate series of freshly taken specimens might be available to make comparisons of real value. During the past fall the author spent two months at the Museum of Comparative Zoology, the American Museum of Natural History and the United States National Museum, comparing all this fresh material with the old faded series. The results prove to be of sufficient importance to warrant the publication of the new light which has been thrown on almost every race south of the United States border. As more detailed information on the breeding behavior of northwestern Empidonaces is imperatively needed and a large amount of fresh, unfaded material is essential to the preparation of an authoritative review of the entire genus, I am not attempting this, but simply giving the results of these studies, as far as they have gone. The new material has necessarily changed some of the concepts of Ridgway (1907: 546-549), but this early revision remains useful, surprisingly so, when one considers the meager and faded material at his disposal. Since his time, Hellmayr (1927: 211) has suggested a conspecific relationship between wrighti, griseus, pulverius, fulvipectus, now known as affinis affinis, and trepidus. Furthermore, he placed salvini and flavescens with the difficilis group. Dickey and van Rossem (1928) objected to this last action. Later, Griscom (1932: 264) definitely dissociated flavescens from difficilis. Further-