Scientific essentialism holds that: (1) each scientific kind is associated with the same set of properties in every possible world; and (2) every individual member of a scientific kind belongs to that kind in every possible world in which it exists. Recently, Ellis (Scientific essentialism, 2001 ; The philosophy of nature 2002 ) has provided the most sustained defense of scientific essentialism, though he does not clearly distinguish these two claims. In this paper, I argue that both claims face a (...) number of formidable difficulties. The necessities of scientific essentialism are not adequately distinguished from semantic necessities, they have not been shown to be necessities in the strictest sense, they must be relativized to context, and they must either be confined to a subset of scientific properties without warrant or their connection to causal powers must be revoked. Moreover, upon closer examination (1) turns out to be a trivial thesis that can be satisfied by non-kinds, and (2) is inapplicable to some of the most fundamental kinds in the basic sciences. (shrink)