SOME EXAMPLES O F VICARIANCE IN CRUSTACEAN COMMUNITIES O F STEPPIC LAGOONS New information on the historical biogeography and on the phenomena on allopatric speciation and dispersion arises from the study of the crustaceans of the steppic Spanish water bodies. In this paper, the communities of phyllopods and copepods of temporary fresh-waters are analyzed. The community structure is similar in steppic environments throughout the world and the regularity observed in the association of certain biological types is remarkable. However the specific composition varies considerably on a regional basis when comparing different areas that are very distant, or even in areas that are closely related biogeographically. In most cases, a common ecological meaning is confered to these communities, and thus the different regional manifestations are interesting examples of vicariance. In the Iberian Peninsula these regional differences are very clear. North to Central System, the communities are characterized by Triops cancriforrnis .sirnple,r, Lepidurus apus and Cji-icus tetracerus. In the south, T. cczncrif?)rrnis rnauritanicus and C'. grzlbei are characteristic. On the other hand, species found in communities of both areas, as Duphniu sirnilis, reveal some morphological differences which can be interpreted as indices of allopatnc subspeciation. The fact that there are endemismes, as is the case of Cjzicus grilhei, and restricted areas of distribution lead one to question the supposed facility of dispersion, which is typically attributed to these organisms because of their capacity to produce forms of resistance. This also supports the hypotbesis of the considerable age of the freshwater steppic Iberian fauna. Therefore the permanente of these organisms through time could find its explanation in the continuous and conditions from the Tertiary to present time rather than in a greater efficiency in the colonizing strategies.