Planktonic foraminifera shell weights have been recognized as possible proxy for surface water carbonate ion concentration [CO sup(=) sub(3)] and atmospheric CO sub(2). However, to utilize this proxy, it is important to understand whether shell weights truly reflect surface water [CO sup(=) sub(3)]. We utilize shell weights of Globigerina bulloides and Globigerinoides ruber in the size range of 300 to 355 mu m from a sediment core recovered from above the lysocline in the upwelling region of western Arabian Sea. Shell weights of G. ruber and G. bulloides show significant correlation with their shell size from recent to 16 kyr, which suggests that shell calcification was controlled by optimum growth conditions. On the other hand, during 16 to 22 kyr, there is no correlation between shell weights and shell size. However, shell weights of G. bulloides exhibit significant negative correlation with annual sea surface temperature which suggests that G. bulloides calcification might have been controlled by surface water [CO sup(=) sub(3)]. Therefore it is suggested here that shell weights of G. ruber and G. bulloides cannot be utilized to reconstruct surface water [CO sup(=) sub(3)] in this region.