The contents of 21 trap-nests located in Sagarmatha National Park, Nepal, in 2002 and 2003 revealed interesting aspects of the biology of Ancistrocerus sikhimensis Bingham (Hymenoptera: Eumenidae). The nests included 2-7 brood cells separated by mud partitions. The dimensions of these structures seem to increase from the first cell to the last one constructed by the wasp. Females always used all the available space of the trap-nests, and the variability in the number of cells per nest essentially depended on their different dimensions. All the emerged adults were females, and we suspect that this species is bivoltine, with a highly shifted sex ratio between the two generations. In 2002, the parasite Chrysis sp. aff. coelestina Klug, recorded for the first time on this host, was responsible for a rate of parasitism per nest of 0%-100%, with an average of 41.65%. A second cuckoo wasp, Chrysis violenta ultramonticola Linsenmaier, emerged from one nest in 2003. Most A. sikhimensis females housed, mainly on the abdomen, hypopi of the mite Vespacarus sp., which is known to be involved in other wasp-mite associations. Unlike other mite-symbiotic eumenid wasps, A. sikhimensis does not present an acarinarium on its body to house the mites.