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Brood parasitism is not well documented in arthropods. We studied interactions among two species of salticid spiders (Habronattus tranquillus and Metaphidippus manni) and a diguetid spider (Diguetia mojavea). Observations and experiments show that these salticids invade a web and deposit their own eggs after D. mojavea mothers die. Although salticid eggs are laid after diguetid eggs are produced. their eggs are larger than those of diguetids, develop much more rapidly, and invariably emerge before the first diguetid spiderlings hatch. Salticid spiderlings also emerge at a significantly larger size than diguetid spiderlings. Thus. the salticid spiderlings have both timing and size advantages over the diguetid hatchlings. In addition. salticid spiderlings prey on resident diguetid juveniles and thereby significantly reduce the host’s fitness. We document experimentally that one defense against this parasitism is the mother’s presence after her egg sacs are deposited. Other possible diguetid defenses include high fecundity and asynchronous hatching of egg sacs. Our results suggest that these salticids are obligate brood parasites of D. mojavea and provide one of the first experimental examples of host-parasite coupling among arthropods.