We consider how the OO notion of subtyping relates to LOTOS testing theory. In particular, we investigate which of the standard LOTOS preorders is a suitable instantiation of behavioural subtyping and argue that each of the main preorders, trace preorder, trace extension, reduction and extension, is in some way deficient. Then, in the light of pre and post condition based models of OO subtyping, we re-work the basic interpretation applied to LOTOS behaviour descriptions. We argue that this re-interpretation enables reduction to be used as an instantiation of behavioural subtyping
