Sixty nine quantitative (100 cu cm) bottom samples from seven modern subenvironments of the Bay St. Louis and Cat Island barrier-island complex, Mississippi, have yielded data of potential use in paleoecology. Analysis of the numerical means of living and total (live plus dead) foraminiferal and ostracodal populations across these seven discrete marginal marine environments (estuarine, bay-marsh, bay, sound, barrier-marsh, barrier-beach, gulf) reveals several interesting patterns. These patterns appear to be local, but are not necessarily applicable throughout the entire detrital province of the northern Gulf. The data indicate that living foraminiferal populations are relatively large and generally increase from estuarine to gulf regimes, reaching maxima on barrier beach s (mean 50.3 percent) and minima in the bay-marsh (mean 2.1 percent). Total test production (live plus dead) decreases along the same transect, reaching maxima in the bay-marsh (mean 1,219 individuals) and minima in the sound (mean 51 individuals). Ostracodal data disclose that living populations of these Crustacea are relatively small and generally decrease from estuarine to gulf milieu, reaching maxima in the estuaries (mean 37.7 percent) and minima in the gulf (mean 0.2 percent). Total carapace production (live plus dead) generally increases along the same transect, with maxima in the gulf (mean 57 individuals) and minima in the bay (mean almost 0). The data suggest that an almost inverse relation exists between living foraminiferal populations and total populations in the same enviro ment, implying that marginal-marine Foraminifera may be poor paleoecologic indicators. On the other hand, the curves comparing total ostracodal populations with their living populations show parallel trends, indicating that marginal-marine ostracods are better paleoecologic indicators than their protistan cousins.