In Cambro‐Ordovician transitional beds of the Hassi R'mel area, northwestern Algeria, three stratigraphically successive acritarch assemblage zones can be recognized: HM/A (uppermost Cambrian), HM/B (basal Tremadoc), and HM/C (lower Tremadoc; with associated graptolites of theRhabdinopora flabelliformissensu lato group). Thus, the boundary between assemblages HM/A and HM/B is considered to approximate to the Cambrian‐Ordovician boundary in the study area. Six morphologically distinctive acritarch species that facilitate recognition of zonal boundaries are described. One genus and four species are newly instituted:Phenacoon imperfectumgen. et sp. nov.,Impluviculus araneussp. nov.,Ladogella saharicasp. nov., andMultiplicisphaeridium attenuatumsp. nov. Revised circumscriptions of two other species—Acanthodiacrodium baculatumandBaltisphaeridium verutum—incorporate morphological details not previously available.P. imperfectumandL. saharicaare abundant at the top of assemblage zone HM/A;A. baculatumcharacterizes the base of assemblage zone HM/B; andB. verutum, I. araneusandM. attenuatumare characteristic components of the lower part of HM/C assemblage zone.