Species delimitation within Euphorbia can be challenging, especially when relying on leaf characteristics.  Leaves of species within Euphorbia sect. Anisophyllum often exhibit leaf asymmetry, anisophylly, and heteroblasty. Differentiating Euphorbia fendleri and E. chaetocalyx relies on the narrowness of leaf shape, but abundant intermediates suggest both species form species complex across the western United States.  Various authors have treated E. chaetocalyx as either a species or simply a variety of E. fendleri.  More recent morphological and geographic evidence suggests that E. chaetocalyx intergrades freely with some populations of E. fendleri but not with others.  The populations of E. fendleri that do not intergrade with E. chaetocalyx appear to differ from intergrading E. fendleri populations by characteristics of leaf asymmetry, anisophylly, and heteroblasty.  I will present the results of a study attempting to delineate species within this complex utilizing leaf asymmetry, anisophylly, and heteroblasty.