A comparative statistical study of foraminiferal and ostracode assemblages in Domerian marls from the Aquitaine basin (France) shows that the taxonomic structure of the populations fluctuates through time. Multiple Correspondences Analyses (M.C.A.) processing presence-absence data (table of taxa in the successive sample levels) were conducted. The first axis (F1) of this M.C.A. is related to faunal diversity and expresses a biotic capacity gradient. The F2 axis coincides with accomodation space and can be used to identify systems tracts and sequence boundaries. The F3 axis is interpreted as representing an oxygenation gradient. Short-term fluctuations at the systems tract or half systems tract level appear in the residuals of regressions and the distance between samples.