Abstract The original description of Halymenia actinophysa was based on few thalli and limited characteristics. Moreover, later descriptions seem to have included more than one species; the confusion of identity was exacerbated by the lack of carposporangial material. From 1995 to 2006, extensive collections of this species were made at several locations (including the type locality) in the southwestern part of the Gulf of California. The study focused on gametangial material. Our aim was to analyze the morphology and anatomy of this species (based on vegetative and reproductive features) to better understand species discrimination. Halymenia actinophysa is a foliose alga with orbiculate to elliptical ruffled blades and a gelatinous, membranous, slippery frond texture. In transverse section, the blade has a thickness of 150–300 μm, a cortex with one or two rounded or elongated cells and an internal layer with elongated, stellate cells connected to the anticlinal filaments of the medulla. Reproductive anatomy is in agreement with the modern concept of the genus and supports the concept that earlier descriptions comprised a mixture of species. The present study strongly suggests the need for a formal review of the genus Halymenia in the Gulf of California to delimit the species and to relate them to other genera of the same family.