ABSTRACT The excretory organs of Branchiostoma (= Amphioxus) are so peculiar, in that they resemble the protonephridia of many invertebrates and differ radically from the excretory tubules of other vertebrates (Goodrich, 1902 and 1909), that it is a matter of some importance to ascertain whether organs of similar structure occur in the allied genus Asymmetron. Andrews, so far as I am aware the only author who has dealt with this genus in detail, failed to find them in the Asymmetron lucayanum he described (Andrews, 1893, p. 229). During a recent visit to Bermuda I obtained specimens of this species,1 and give here the results of the study of fresh and preserved material carried out in the Bermuda Biological Station during last July and August and since in Oxford. A comparison is also made with the excretory organs of Amphioxus.