Horizontal starch gel electrophoresis was used to estimate isozyme allele frequencies at 13 enzyme loci for each of 1048 northern red oak embryos representing 8 populations in Pennsylvania. The average expected heterozygosity over all loci and populations, HT;, was 0.233. Red oak populations were in Hardy Weinberg equilibrium (FIS = -0.008) and very weakly differentiated (GST = 0.009). In two populations the maximum likelihood multilocus estimates of outcrossing were 1.002 and 1.015. Neither estimate was significantly different from complete outcrossing (t = 1.0). Although 28 of 32 single-tree outcrossing estimates were not significantly different from t = 1.0, an analysis of singletree outcrossing rates indicated significant tree-to-tree variation in both stands. Only 2 of 24 heterogeneity G-tests were significant, indicating that pollen pool allele frequencies showed little spatial variability and/or outcrossing was independent of maternal genotype. These results suggest that (i) family clustering is weak in red oak populations, and (ii) most members of open-pollinated families are half-sibs probably derived from a large number of pollen parents. Thus, results of open-pollination progeny tests are likely to reflect real differences in mean breeding values of most families. In addition, estimates of additive genetic variances and heritabilities are not likely to be biased sharply upwards by an increased genetic correlation among family members. Furthermore, acorns derived from seed production areas are likely to be outcrossed and contain a high level of genetic variation. For. Sci. 37(5):1376-1389.