Key to Order Isopoda

Taken primarily from  Kozloff, 1987, 1996  p. 330 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

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1a	Adults parasitic on other Crustacea (Cirripedia, Mysidacea, Decapoda): females generally grotesque, often asymmetrical, with most appendages reduced or absent; males small, symmetrical (a male is usually associated with each female)	Suborder Epicaridea
1b	Adults free-living, parasitic on fishes, or commensal with other isopods; females and males recognizable as normal isopods, even if specialized for clinging to fishes	2
2a	Uropods ventral to the pleotelson  (photo) or decidedly lateral to the pleotelson	3
2b	Uropods terminal, close to the midline	5
3a	Uropods ventral, forming a valvelike covering over the pleopods (photo)	Suborder Valvifera
3b	Uropods lateral, flattened, and, together with the pleotelson, forming a caudal fan	4
4a	Body at least 7 times as long as wide, nearly cylindrical; exopodites of uropods arching over the pleotelson; pereopod 1 larger than the others and subchelate (pereopods 2 and 3 may also be subchelate)	Suborder Anthuridea
4b	Body usually not more than 5 times as long as wide; exopodites of uropods not arching over the pleotelson; pereopod 1 neither especially enlarged nor subchelate (except in the genus Tecticeps, in which it is subchelate)	Suborder Flabellifera
5a	Body slender, at least 7 times as long as wide; pleon with 2 free pleonites plus the pleotelson, these 3 units being nearly equal; pereopod 1 subchelate: generally under 2 mm long; living between sand grains	Suborder Microcerberidea
5b	Body usually robust, much less than 7 times as long as wide; pleon not consisting of 3 nearly equal units (if there are 3, the pleotelson is much larger than the free pleonites); pereopod 1 generally not subchelate (but sometimes subchelate); mostly more than 2 mm long (but some species may be smaller); in various habitats	6
6a	Aquatic; pleon generally with not more than 2 free pleonites (sometimes 3) plus the pleotelson; both antenna 1 and antenna 2 well developed	Suborder Asellota
6b	Essentially terrestrial, although certain species are restricted to moist situations near the high tide line; pleon generally with 5 free pleonites plus the pleotelson; antenna 1 rudimentary, with only 2 or 3 articles	Suborder Oniscoidea

  Suborder Epicaridea
  Suborder Flabellifera
  Suborder Valvifera
  Suborder Oniscoidea