Trichilogaster is a small genus of chalcid wasps in the family Pteromalidae, subfamily Ormocerinae. With one described exception, they all are Australian species that are gall-formers on Australian species of Acacia. The exception is an Arabian species. Apart from its ecological interest, the genus is of practical importance because some of its members are successful biocontrol agents in South Africa at least, where T. acaciaelongifoliae and T.