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Females of the feather-legged spider Uloborus plumipes invade, and compete for, each other's orb webs. In the context of these competitive interactions the question arose how the spiders communicate. Since substrate-borne vibrations are the most important component of the sensory environment of web-building spiders, we investigated vibratory movements that might serve as signals of communication. Three behaviors were found to be associated with female–female contests and to cause propagating vibrations in the spider webs: thread pulling, abdominal trembling, and web shaking. While thread pulling and abdominal trembling were also observed when prey insects were caught in the webs, web shaking occurred only in response to the presence of a competing conspecific. Caused by flexing of the first legs and a vigorous rotary movement of the opisthosoma, web shaking creates a short burst of strong oscillations of the orb web. This behavior always elicited a behavioral reaction by the competitor and may serve as an intraspecific signal in the mutual assessment of competing spiders. We suggest that web shaking communicates resource holding potential in U. plumipes.