DIFFICULTIES in identifying certain Empidonax flycatchers recently led me to survey the literature concerning the molts and plumages of birds of this genus and to examine a large number of specimens. In the course of this inquiry it became evident that there has been much confusion in regard to the molts and plumages of some of the species and that the literature is often misleading or actually incorrect. In the cases of the Acadian Flycatcher, Empidonax virescens, and the Yellow-bellied Flycatcher, E. flaviventris, among others, this situation can lead to some difficulty in the allocation of specimens, and it is with these two species that this paper deals. Griscom (1923: 234) emphasized the close similarity of Acadian, Least (E. minimus) and Alder (E. traillii) flycatchers in the fall, but it seems not to have been generally recognized that any special difficulty attends the separation of any of these from the Yellow-bellied Flycatcher. Actually, during the fall a substantial proportion of virescens closely resembles and can be most perplexing for this reason, especially to those not familiar with the genus. The possibility of misidentification is suggested by the fact that the only extant specimens of flaviventris for South Carolina, taken and recorded by the veteran ornithologist Arthur T.Wayne, were referred to virescens upon subsequent re-examination by Allan R. Phillips and E. B. Chamberlain (Sprunt and Chamberlain, 1949: 354). Inadequate description of the plumages of the Acadian Flycatcher is largely responsible for confusion such as this. I first became aware of this problem when H. B. Tordoff and I attempted to identify a number of specimens we had collected near Henderson, Kentucky, in early September, 1949. We found that in a series of six specimens, which we had assumed in the field to be flaviventris, there were actually some individuals of virescens. Nowhere in the literature could we find reference to such a flaviventrislike plumage of that species. My material, supplemented by additional specimens in the University of Michigan Museum of Zoology (hereafter referred to as U. M. M. Z.), fortunately contains a nearly complete molting series of virescens, permitting more thorough analysis of its molts and plumages than has hitherto been possible.