A series of large Cubomedusae from northern Australian seas-Darwin and north Queensland-has been examined. These are of a single species, described here as Chironex fleckeri, gen. nov., sp. nov. Morphological and systematic studies have been made. It is shown that the system of gonads is markedly different from that obtaining in other Cubomedusae, in which the gonads are leaf-like lamellae that arise at the interradial septum and project into the perradial stomach pouches. In Chironex, only a narrow ridge is present at the interradial septum in the same situation, at most about 0.4 mm wide, and the gonads are lobulated structures that arise alongside the perradial suspensoria, high in the bell cavity, and project into the bell cavity. This structure is also present in the genus Chiropsalmus, but in more primitive form, and in the past has been called the "gastric saccule", of unknown function. It is apparently absent (judging by the literature) in other Cubornedusae. The histology of the gonads and other relevant features in Chironex fleckeri are described. The term "superior gonad" is introduced for what has hitherto been called the "gastric saccule", and the term "lateral gonad" is used for the structure arising at the interradius in the perradial stomach pouches. The systematics of the Cubomedusae are reviewed in the light of the differences in the gonads. The family Chironecldae, fam. nov., is raised for Chironex, gen. nov. The genus Chiropsoides, nom. nov., is proposed for Drepanochirus Krumbach, 1925 (non Pringuey 1901 preoc.), and hence Chiropsoididae, nom. nov., for the family Drepanochiridae Krumbach, 1925. The clinical effects of the stingings of Cubomedusae on man are considered briefly, where relevant. It is probable that Chironex fleckeri is responsible for severe stingings and deaths of humans in northern Austrahan waters, but dlrect proof of this hypothesis is lacking.