When teaching children part terms, adults frequently outline the relevant part rather than simply point. This pragmatic information very likely helps children interpret the label correctly. But the importance of gestures may not negate the need for default lexical biases such as the whole object assumption and mutual exclusivity. On this view, children initially assume that a novel label refers to a whole object. If the label for the object is familiar, mutual exclusivity blocks this assumption and frees children to look for a part referent. In this study, the authors taught children part terms by outlining a novel part of a real object. We made mutual exclusivity available by showing children familiar whole objects with novel parts and unavailable by showing unfamiliar whole objects with novel parts. During test, the object was oriented with the part facing away from the child to distinguish pointing to the object from pointing to the part. Both 2-year-olds and 3-year-olds learned more part labels when mutual exclusivity was available. Thus, mutual exclusivity is indispensable even when part labeling is accompanied by naturalistic communicative gestures.