ABSTRACT--Coreopsis nuecensoides and C. nuecensis are narrowly distributed endemics of southeastern Texas.  While they overlap in range, they differ in chromosome number, and F₁ hybrids exhibit strong sterility barriers.  Previous morphological, cytogenetic, and allozyme studies suggested that Coreopsis nuecensoides and C. nuecensis are very closely related members of a progenitor-derivative species pair.  The two species differ substantially in terms of their leaf flavonoid chemistry, but the flavonoid data do not address the exact nature of the relationship between them.  We have further examined the relationship between the species by analyzing genetic diversity within both species using chloroplast DNA (cpDNA) restriction site data.  Sixteen restriction site changes were used to define thirteen distinct cpDNA haplotypes.  The pattern of relationships among haplotypes provides some support for the presumed ancestor-descendent relationship, but other interpretations are possible.  Only one cpDNA haplotype was shared by the two species; of the remaining twelve, seven were unique to one species and five to the other.  This result is consistent with results of flavonoid studies, in which each species exhibited flavonoid compounds not seen in the other, but differs from the allozyme results, in which the vast majority of alleles were shared by both species and only the presumed progenitor exhibited unique alleles.  Taken together, the data support a very close relationship between the species, but are equivocal with regard to a progenitor-derivative relationship.