We describe here rapid gamete growth and artificial fertilization method of species of the larvaceans (appendicularians), Oikopleura dioica and O. longicauda (Family Oikopleuridae: Class Appendicularia: Subphylum Urochordata). In these species, oocytes grew very rapidly from about 40 μm in diameter to about 75 μm (O. dioica) and 110 μm (O. longicauda), respectively within a few hr. Moreover, cutting off the gonads at the last phase of the growth stage yielded matured gametes. The eggs and sperm obtained by the dissection of gonads could be fertilized when they were mixed together.