Rhynchocalycaceae is a monotypic family represented by the rare Rhynchocalyx lawsonioides from South Africa. Although unquestionably myrtalean, it is isolated in the order with closest affinity to Alzatea verticillata in the monotypic Alzateaceae. Foliar flavonoids of Rhynchocalyx are reported for the first time. The flavonoid pattern comprises five quercetin 3-0-glycosides, present in approximately equal concentrations. These are quercetin 3-0-glucoside, 3-0-diglucoside, 3-0-rhamnoside, 3-0-xyloside, and 3-0-galactoside. Quercetin 3-0-glucoside and quercetin 3-0-diglucoside also occur in Alzatea. The pattern agrees with the typical profile of the Myrtales in which flavonols are common and flavones are rare. It differs in absence of myricetin, which is frequently found in the order. No more specific relationships are possible based on these flavonoid data due to the generalized nature and widespread occurrence of the compounds. Rhynchocalycaceae is a newly recognized monotypic family based on the rare Rhynchocalyx lawsonioides Oliv. from Natal, South Africa. It has been regarded generally as a genus of the Lythraceae, related to Lawsonia in subtribe Lagerstromiinae (Oliver, 1895; Sprague & Metcalfe, 1937) although that position was rejected by the monographer of the family (Koehne, 1903). Recently, it was included in a remodeled Crypteroniaceae as sole member of the subfamily Alzateoideae, tribe Rhynchocalyceae (van Beusekom-Osinga & van Beusekom, 1975). Anatomical, embryological, and morphological data now support the relationship of Rhynchocalyx to the unigeneric Alzateaceae but reflect a degree of isolation that merits its recognition as a separate family (Johnson & Briggs, 1984; Graham,