Starch gel electrophoresis was utilized to estimate the amount of genetic divergence at 16 putative isozyme loci among populations of six native red oak species occurring in New Jersey: Quercus coccinea, Q. ilicifolia, Q. marilandica, Q. palustris, Q. rubra, and Q. velutina. The mean genetic identity for pair-wise comparisons of 15 populations of the six species was 0.876. This value increased to 0.958 upon exclusion of populations of Q. palustris, indicating a high degree of allozymic similarity among the remaining species. Gene diversity analysis demonstrated that most of the variation present in Quercus populations exists within populations. Additionally, standard measures of genetic variability obtained from this limited number of Quercus populations were concordant with reported and predicted estimates from widespread outcrossing plants that exhibit similar life history and ecological traits. With the exception of Q. palustris, the isozymes examined are of little value in identifying species since marker alleles were not found. However, differences in gene frequencies at polymorphic loci were detected. Patterns of genetic differentiation did not correlate with the five classical taxonomic series delimiting these species suggesting that the series do not represent genetically differentiated groups of species excluding series Palustres (Q. palustris). It is also suggested that speciation among the five closely related red oak species was rapid and apparently occurred with minimal changes in the genes coding for the isozymes examined. These preliminary data document detectable genetic divergence in at least one species within the red oak species complex.