in this work it is proposed that the conditions of identity for biological organisms are given by the following principle: for all organisms x and y, x = y if and only if x has been caused by the self-preserving activity of y. This principle determines both the inter-temporal identity of organisms and the identity of organism in different possible worlds. It unifies what can be supposed about conditions of identity coming from-at least-three different conceptions about the nature of biological organisms, those of RobertWilson, Thomas Pradeu and Ellen Clarke. These conceptions are presented and compared.