Valvilli, articulated chitinous flaps on the lower ovipositor and sting valves in certain Hymenoptera, were studied using light and electron microscopy. Presence or absence, number, position, and structure of valvilli in the ovipositors of more than 300 species distributed among 31 families (including representative of 74 subfamilies of Ichneumonoidea) have been examined. Valvilli were found to occur in most Ichneumonoidea and most families of the aculeate lineage. Valvilli were absent from the Symphyta, Trigonalyoidea, Stephanoidea, Evanioidea, Cynipoidea, Chalcidoidea, Proctotrupoidea, Megalyroidea and Ceraphronoidea. Aculeates have a complex, double valvillus, which differs considerably in structure from those found in the Ichneumonoidea. Within both the Ichneumonidae and Braconidae the number and the position of the valvilli appear to be phylogenetically significant. Within the Braconidae, in particular, valvillus structure varied between subfamilies. The implications of valvillus distribution and structure in the Hymenoptera as a whole and within the Ichneumonoidea are discussed.