Synagris wasps (Hymenoptera: Vespidae: Eumeninae) include some of the most spectacular examples of exaggerated secondary sexual “weapons” known. I examined the scaling relationship between tusk length and head width in S. cornuta, a widely distributed, Afrotropical species. Data from museum collections and field observations demonstrate that, unlike most aculeate Hymenoptera, males of S. cornuta are larger, on average, than females and are dimorphic for mandibular tusk length with a discontinuous scaling relationship shape for head width. Other measures of body size are also examined. I also studied female nesting behavior and male behavior in the field in Côte d'Ivoire, West Africa. Nests and nest construction and other female behaviors are described. Most nests were constructed on the undersides of leaves of Aframomum latifolium Afz. (Zingiberaceae) using soil from termite tunnels or old S. cornuta nests moistened with water from leaves. As reported by previous authors, females progressively provision larvae and macerate pieces of caterpillar before feeding it to larvae. Larval silk attached to the leaf through an opening left by the mother may extend the lifetime of nests. Most nests included 1–3 cells and may be limited in size due to weight on leaves. Occupants of a high proportion of nest cells failed to emerge due to parasitism by rhipiphorid beetles and loss of nests. Males displayed trap-lining behavior, patrolling from nest to nest. Some tusked males guarded nests for periods of hours a day over several days. Males used their tusks in male-male competition over mud nest cells containing females about to emerge and also wielded them conspicuously in threat displays. Other males remained nearby and flew up to confront guard males. Unarmed (tuskless) males are less common and were not observed to guard. Some Synagris species show large weapons that are probably intrasexually dimorphic, while others simply show unusual mandibular development and still others lack weapons entirely. Depending on species, males are smaller than, equal to, or larger than females.