The trochophore larva of Harmothoe imbricata was examined by both light and electron microscopy. Light microscopy of living trochophores was limited to a study of the action of cilia and muscles, and observations on trochophores at various stages from the egg to the fully formed trochophore. The observations on the developing trochophore revealed the presence of apparently uniquely acting cilia. Fixed, sectioned trochophores were examined under the light microscope as an adjunct to electron microscopy to give the gross anatomy. Scanning electron microscopy was used to determine the arrangement of cilia and glands on the surface of the trochophore. This demonstrated that previous views, at least on the apical array of cilia, were erroneous, and that there is considerable asymmetry in the positions of both cilia and gland pores. Transmission electron microscopy of the fully formed trochophore afforded the first information at an ultrastructural level on all the organ systems of a trochophore, and revealed the presence of previously unknown structures such as modified cilia and a probable photoreceptor which is additional to the pigmented eyespot. Certain general issues of contention in electron-microscopical work were able to be resolved in the course of this work. This included the finding of neuro-ciliary synapses, and certain significant features in the development of trochophores from the egg to the fully formed trochophore. The suitability of this animal for experimental investigation became clear as the fine structural analysis proceeded. Accordingly, preliminary studies were made on pepsin digestion of thin sections and the testing of drugs on whole- animals before fixation.