Coreopsis basalis and C. wrightii are diploid, annual plants centered in cast and southeastern Texas, with the former introduced into the southeastern United States. The two species are very similar morphologically but are highly intersterile due to differences in chromosome structure. Electrophoresis determined the amount of divergence between the two taxa at 14 genetic loci coding soluble enzymes. Mean genetic identity for ten populations of C. basalis is 0.94 and the value for four populations of C. wrightii is 0.97. Average genetic identity for populations of the two species is 0.92, indicating little if any genetic divergence between the species as compared to conspecific populations. The enzyme data and morphology are concordant with the hypothesis that speciation occurred via chromosomal repatterning with minimal divergence at genes coding for isozymes. Isozyme data support the hypothesis that C. basalis and C. wrightii are more closely related to each other than either is to the other two annual species of sect. Coreopsis, C. nuecensoides and C. nuecensis. The mean genetic identities for C. basalis-C. nuecensoides, C. basalis-C. nuecensis, C. wrightii-C. nuecensoides, and C. wrightii-C. nuecensis are 0.77, 0.79, 0.73, and 0.76, respectively.