Studies of modern foraminiferal ecology have provided at least 5 distinct criteria for the reconstruction of marine paleoenvironments: 1) both the number of species and specimen abundance increase away from shore and with increasing depth of water to maximum values on the outer shelf and in the upper and middle bathyal zone; 2) diverse porcelaneous species are abundant in shoal nearshore marine environments; 3) arenaceous Foraminifera with simple interiors may be abundant in shallow waters whereas more complex types with labyrinthic interiors are more characteristic of bathyal depths; 4) deposition of planktonic species occurs most abundantly on the outer shelf and in the upper bathyal zone, with even greater abundances in deeper waters under the right conditions; and 5) similar environmental adaptations of modern species and fossil homeomorphs (and isomorphs) may be assumed, especially for groups of species. Faunal contamination or mixed faunas must be recognized as an ever-present deterrent to valid stratigraphic and paleoecologic analyses. One important type of contamination is the displacement of shoal faunas into deeper water; however, the displacement problem is minimized by selecting the deepest bathymetric indicator species in each sample. Another type of mixed fauna is produced by the reworking of faunas from older strata into younger sediments. This type of contamination usually is recognizable by differences in preservation or incongruous mixtures of index species of different ages or both. To test the value of foraminiferal ecology to the finding of oil deposits, a detailed study was made of the biofacies of the middle Tertiary of the San Joaquin Valley, California, The analyses of more than 5,000 samples (cores) made it possible to construct a series of paleobathymetric maps showing the gradual evolution of the San Joaquin basin. Examples of this study are presented to illustrate the methods of applied paleoecology and their implications.