The present paper deals with the results of observatons of zoeal development of the subfamilies Iliinae and Leucosiinae (family Leucosiidae), using the following five species as materials : Arcania undecimspinosa elongata YOKOYA, Myra fugax FABRIClUS, Philyra syndactyla ORTMANN, Philyra pisumde HAAN, and Leucosia longifrons de HAAN. The general features of these species are, (a) antenna II and telson fall in types D and E respectively (AIKAWA 1929) , (b) endodop of maxilliped I has setae arranged as 4・1-2-1-2-2, (c) setation on protopod of maxilliped II is 1・1・1・1(4), (d) exopods of maxillipeds bear 6 natatory setae in stages II and III, (e) a pair of lateral knobs are found on abdominal somites II and III, and (f) articulation of telson is not can be found through all stages. The differences between the two subfamilies may be seen in the zoeal stages ; armature of maxilla I and II, and length of three pairs of inner setae on caudal furcae. Specific defferences in the characters of zoeae are as follows : carapace of Philyra pisum has a rostral spine, no dorsal and lateral spines, that of Philyra syndactyla has rostral and dorsal spines, but other species have these three kinds of spines. In Arcania undecimspinosa elongata, protopod of maxilliped I lacks chromatophore which is found in other species. Endopod of maxillipede II of Myra fugax consists of two-segments in stages I and II, whereas other four species have one or two-segments in all stages. Leucosia longifrons has the setation of 2・3・2・1(8) on protopod of maxilliped I, while other species have 2・2・2・2(8) setation respectively.
