It is commonly thought that framework reefs did not exist prior to Middle Ordovician time. Lower Cambrian archaeocyathan-algal limestones from several localities suggest that this view may be incorrect. In particular, limestones in the Poleta Formation of western Nevada and eastern California display a complex branched framework and a biological zonation similar to that described from Ordovician and younger rocks. These archaeocyathan-rich limestones are associated with coarse-grained oolite, contain tidal channels, and are flat-topped and laterally continuous rather than biohermal. I interpret them to have been wave-resistant, environment-dominating, framework reefs.