The present study focusses on morphological flower characters considered crucial in the supposed transition from Horridocactus Backeberg to Neoporteria Br. & R. sensu Backeberg. Additionally, physiological characters such as photonastic/ scotonastic perianth segment movement and photoperiodism were documented and found to be informative at species level. Differing dimensions of flowers and their critical relationships to other characters, as well as different flowering periods, separate the southern from the northern taxa. In combination with results from a study based on anatomical data in Eriosyce sensu lato published previously by Nyffeler & Eggli (1997), these findings provide strong support for a separation of Eriosyce subgibbosa ssp. clavata (Söhrens ex Schumann) Kattermann from E. subgibbosa (Haworth) Kattermann, and the necessary nomenclatural change is provided. Additionally, several synapomorphies and phytogeography suggest that E. subgibbosa ssp. wagenknechtii (Ritter) Kattermann, ssp. vallenarensis (Ritter) Kattermann and E. subgibbosa ssp. subgibbosa var. litoralis (Ritter) Kattermann (= the former N. wagenknechtii Ritter, N. vallenarensis Ritter and N. litoralis Ritter) seem to be more closely related to the northern species E. villosa (Monville) Kattermann than to the southern E. subgibbosa. When bioclimatic and phytogeographical data were mapped onto the tentative phylogenetic tree resulting from the analysis of the floral characters, these taxonomic conclusions received even stronger support. Eriosyce subgenus Neoporteria was originally published invalidly (Hoffmann & Walter, 2004), a situation which is rectified here by its formal publication. E. clavata ssp. nigrihorrida (Backeberg) H. E. Walter is also presented as a new combination. The position of two closely related taxa of subgenus Neoporteria, E. taltalensis (Hutchison) Kattermann and E. chilensis (Hildmann ex Schumann) Kattermann, is discussed, but their position remains unresolved. The role in evolution played by pollinators, particularly hummingbirds, is discussed.