Predatory habits of apoid wasps are well documented for many species, revealing a choice of prey ranging from generalist to specialised, but few studies tested the degree of specialisation when compared with the availability of prey in the environment. In a study carried out in northern Italy, nests of the mud-dauber wasps Sceliphron spirifex L. and S. caementarium Drury were collected to obtain the spider prey of the wasps, and a survey of the nesting area was performed to ascertain frequency of the available spider prey species in the environment. Wasps preyed preferably upon spiders of the family Araneidae. Adult preferred spider prey size ranged from 4 to 6 mm in length. The factor which most affected prey selection was the ecology of the spiders, with orb-web spiders being the preferred prey despite the fact that terricolous, non-web groups were the most abundant in the locality. Sex (female, male or juvenile) of prey was also important in prey selection: juvenile spiders were the most preferred even though males and females were equally and most abundant (respectively). Sceliphron spp. seem almost to be specialised rather than generalist predators. These results suggest that the terms 'generalist' or 'specialised' should not be applied to predators solely on the basis of prey collected from wasp nests, but should also be related to local prey availability.