General interest in the problems of the larval biology of marine animals has increased greatly in the last few years. The importance of factors affecting the size of larval populations has now been recognized in its proper relationship to the size of adult populations. THORSON'S 1946 monograph on the larvae of Danish benthic invertebrates first brought into focus the size of the problem in accurately assessing the "biology" of planktonic larvae. Since then, numerous papers have appeared describing larval phenomena in various restricted groups. In the present study, the author has attempted to investigate, on a specific level, some of the factors involved in determining length of larval life (excepting predation), metamorphosis, feeding and growth in the nudibranchiate gastropods. The larvae of nudibranchs have not until recently received much attention. Thorson considered them only briefly and only two later authors have studied nudibranch larvae. Thorson's conclusions, based mainly on considerations of larval anatomy and relative seasonal abundance in the plankton, were that nudibranch veligers were, in general, non-feeding, non-growing, short term plankters. More recently THOMPSON (1958) found that the larvae of Adalaria proxima were indeed capable of feeding, but could metamorphose without feeding. In another paper, THOMPSON (1959) examined the feeding mechanisms of nudibranch larvae and found that in most animals investigated ingestion and assimilation of food did occur. RASMUSSEN (1952) had already noted apparent growth in the larvae of Favorinus branchialis taken from the plankton near Copenhagen. This study attempts to clarify some of these relationships, particularly the feeding-growth-length of larval life interaction. The author was fortunate to be able to work at the Marine Biological Laboratory, Helsing0r, with essentially the same fauna as that of Thorson.