The behavioural ecology of ichneumonid wasps that attack aculeate Hymenoptera is still largely unknown. Field observations and morphological analyses were devoted to investigate host detection and rate of parasitism by Acroricnus seductor (Scopoli), a natural enemy of the black and yellow mud dauber wasp Sceliphron caementarium (Drury). At the study site, about half of the host nests suffered parasitism by A. seductor. No significant difference was found between the rate of parasitism in sheltered (inside human building) and unsheltered (outside building) nests. Larger nests did not suffer a higher rate of parasitism, and larger brood cells were not more likely to be parasitized. As revealed by contents of parasitized cells, A. seductor appeared to act as a kleptoparasitoid, devouring spider prey and young host larvae. Analysis of video recordings obtained in the field revealed the basic behavioural sequence of host detection. Acroricnus seductor female taps with the antennae the host nest surface and, once a suitable host brood cell is found, inserts the ovipositor through the mud wall, possibly facilitated by the secretion of a mud-softening substance. Behavioural data, together with the presence of modified tips on the apex of female antennae and the relative thicknesses of female fore tibiae, strongly suggest that A. seductor uses echolocation to detect the host.