the adult bopyrid. The only hypothesis that seems reasonable is that sex is genetically determined in the cryptonisci of P. expansus. However, after examination of over 400 Epipenaeon ingens Nobili, only 1 juvenile female bopyrid was removed from an adult E. ingens. On morphological grounds, this juvenile proved to be P. expansus suggesting juvenile association with an adult bopyrid is not widespread even in sister genera. When describing P. expansus Bourdon (1979, Bulletin du Museum d'Histoire Naturelle du Paris 4e Ser. 1, A(2): 471-506) also noted the presence of bopyridia attached to the adult bopyrid. After e adult bopyrid. The only hypothesis t at eems 5 yr of study on a closely related species Parapenaeon japonica (Thielemann), including histological examination of juvenile bopyrids, Hiriawa (1936, Journal of Science of the Hiroshima University, Series B. Div. 1. 4: 101-141) concluded sex was genetically determined in that species. Reinhard (1949, loc. cit.) argued against this view and proposed the generalized account accepted today. We feel there is enough evidence to suggest in the Bopyridae there may be at least 2 types of sex determination, (1) epigenetic determination by host substrate and (2) genetic determination at fertilization as in P. expansus.