On 27 May 1978, while looking for birds in the White Mountains near Tollhouse Springs, Inyo County, California, I located an active Blue-gray Gnatcatcher (Polioptila caerulea) nest which was placed approximately 1 m high near the top of a low sagebrush (Artemesia tridentata) shrub. While Judy Atwood, Thomas LaRoque and I were observing the female gnatcatcher on the nest, we were rather startled to see a female Calliope Hummingbird (Stellula calliope) fly to the gnatcatcher nest and, while hovering, remove nesting material from the outside of the nest cup. Although the female gnatcatcher appeared to watch the pilfering hummingbird during the several seconds it was present at the nest, the gnatcatcher continued to incubate and made no effort to drive the hummingbird away. The male Blue-gray Gnatcatcher was not observed near the nest until several minutes after the hummingbird had departed. We flushed the female gnatcatcher from her nest and found three eggs in it.