Geochemical records from planktonic foraminifera are among the few means available to infer past changes in the upper ocean, although interpretations can be confounded by variability in foraminifera habitat depths. The principal environmental factors affecting foraminiferal vertical distributions were examined through 28 vertically stratified plankton tows taken off southern California in winter, spring, and summer. Each species has a dynamic range of preferred habitat depth, from the mixed layer to the thermocline, characterized by a unique relationship with environmental properties. Globigerina bulloides, Neogloboquadrina pachyderma (dextral), and Neogloboquadrina dutertrei can live near the surface but frequently live deeper as stratification increases and a particular isotherm deepens. Globigerinoides ruber and Orbulina universa vary more closely with the depth of the thermocline than the depth of a given isotherm. The shifts in habitat depth are of sufficient magnitude to affect δ18O records. The effects of such shifts are observed for G. bulloides and N. pachyderma in Santa Barbara Basin sediments. Geochemical signatures from multiple species of foraminifera, each with a different predicted response to environmental change, can permit the detection of changes in habitat depth of a given species and thus more accurately indicate past changes in hydrographic structure.