The biological response to extreme temperatures and salinities is investigated for Globigerinoides sacculifer, Globigerinoides ruber, Globigerinoides conglobatus, Globigerinella siphonifera, Orbulina universa, Neogloboquadrina dutertrei and Globorotalia menardii. Measured in vitro temperature ranges compared well with the global temperature distribution patterns of these species, suggesting that this parameter plays a major role in their biogeographical distribution. The salinity ranges tolerated in cultures exceed the range encountered in modern oceans. Thus salinity does not limit the distribution of these species. -from Authors