We assessed whether CaCO3 concentration of seawater may be relevant to the occur- rence of members of Corallimorpharia and Scleractinia, which are very similar except for the posses- sion by scleractinians of a calcareous skeleton. In collections of both the Challenger Deep-sea Expe- dition 1872-1876 and the US Antarctic (Research) Program, average depth of occurrence was significantly greater for corallimorpharians than for scleractinians. We also compared depth of occur- rence relative to the position of the aragonite saturation horizon (ASH) at many localities from which specimens were collected. Nearly 25 and 50% of stations at which scleractinians were collected were below the ASH for the Antarctic and Challenger stations, respectively; 50 and 100% of the Antarctic and Challenger stations at which corallimorpharians were collected were below the ASH, respec- tively. Statistical analyses of these data to test whether there is a difference in the depth, relative to the ASH, at which scleractinians and corallimorpharians occur indicate a difference for the Chal- lenger but not the Antarctic stations; more data are needed. The scleractinians that tolerate living below the ASH belong to a minority of the genera recorded in the surveys, and do not include spe- cies considered important in forming bioherms; those that occur deepest are solitary. Some deep-sea scleractinians may be unaffected by shoaling of the ASH that is predicted across all ocean basins in the near future, some may be confined to water shallower than is now the case, and others may cease producing a skeleton, becoming morphologically indistinguishable from corallimorpharians.