The diet of the blackwidow spider Latrodectus lilianae (Araneae: Theridiidae) was studied in an arid shrub-steppe of south-eastern Spain during 1990–93. For the first 3 years, prey mummies attached to spider webs were collected once the spiders' activity period was finished, at the end of September. In the fourth year, webs were marked and checked bi-weekly from April to September during the activity period of the blackwidow spider. We identified 2106 prey items from 164 webs. Most prey were Tenebrionidae and Oniscidae, which together represented from 65.4% to 82.0% in frequency and from 78.6% to 92.0% in biomass of all prey. An analysis of selectivity comparing prey availability from prey collected in pitfalls and those collected in the webs showed that spiders preferred Tenebrionidae and Oniscidae. However, an analysis of prey size reflects also that spiders mostly selected prey ranging from 12.5 to 22.5 mm long, suggesting that prey size, rather than taxon, determines prey capture. Other predatory arthropods, as well as five small lizards, were found as prey of the blackwidow spider. This suggests that the blackwidow spider may play an essential role in regulating the food web and trophic structure in this community by preying on other species that potentially compete with or prey upon blackwidow spiders.