INTRODUCTION. A complete description of the summer plumage of Tringa canutus never seems to have been given. For this reason alone it might be of some value to describe it in more detail than has been done before. The chief object of the present paper, however, is not the description of the summer dress, but the discussion of a problem connected with it, which is of much more general interest. As the same problem presents itself in the summer plumage of one of the Knot's nearest relatives, the plumage of this bird, Tringa crassirostris, will be dealt with in the latter part of the present paper. I have to thank my friend Dr. H. Boschma for his valuable assistance in the preparation of this paper. III. THE PLUMAGE OF TRINGA CANUTUS L. 1. MATERIAL. The material on which this investigation is based consists of about 140 skins in the collections of the Zoological Museum at Copenhagen and of the Museums of Natural History at London and at Leiden; besides, a few skins from the collections of Mr. G. A. Brouwer and myself and that of the Zoological Museum at Amsterdam have been studied, making a total of about 150. I have to thank the authorities of the institutions mentioned for giving me kind permission to work in their departments: Mr. R. Hörring, mag. sc, of Copenhagen, Dr. P. R. Lowe of London, Prof. E. D. van Oort of Leiden, and Dr. L. F. de Beaufort of Amsterdam. Most valuable for my purposes were the large number of Iceland and Greenland birds in the Museum at Copenhagen and many birds from America and East Asia in the Museum at London.