In recent years the external morphology of marine choanoflagellates has been the subject of close scrutiny. This is a result of the attention given to marine nanoplankton, of which choanoflagellates are a part, and the relative ease with which choanoflagellates with loricae of costal construction can be observed in shadowcast whole mounts with an electron microscope. However, detailed ultrastructural studies of choanoflagellate protoplasts have been limited to one freshwater species Codosiga botrytis (Ehr.) Saville-Kent (Petersen & Hansen, 1954; Fjerdingstad, 1961; Hibberd, 1975) and four marine species Salpingoeca pelagica Laval (Laval, 1971), Stephanoeca diplocostata Ellis (Leadbeater & Manton, 1974), Savillea micropora (Norris) Leadbeater (Leadbeater, 1974) and Codosiga gracilis James-Clark (Leadbeater & Morton, 1974b). Of these only the recent studies by Hibberd (1975), Laval (1971), Leadbeater & Manton (1974), Leadbeater (1974) and Leadbeater & Morton (1974b) contain micrographs of material processed by modern methods.