Native Danish oak stands are fragmented and decreasing because of the extensive use of

foreign seed sources. Therefore, the population structure of natural Danish oak stands was

analysed by means of six polymorphic enzyme loci. A total of 17 stands of Quercus robur L.

(including an ‘‘outgroup'' sample from The Netherlands), seven Quercus petraea [Matt.] Liebl.

and two putative hybrid stands were included in the study. The average genetic diversity was

similar for the two species as well as for the putative hybrid stands: 0.25 for Q. robur, 0.27 for

Q. petraea and 0.26 for the hybrid stands. The genotypic proportions at two (Pgm and Mnr)

. and 0.26 for the hybrid stands. The genotypic proportions at two (and )

of the six loci showed many signi. cant deviations from Hardy-Weinberg expectations, always

with an excess of homozygotes, whereas the remaining four loci accorded to Hardy-Weinberg

proportions, suggesting a low level of inbreeding. The differentiation of the Q. petraea and Q.

robur populations was quanti. ed with Wright's F-statistics. The within-species component was

populations was quanti. ed with Wright's -statistics. The within-species component was

low, 0.022, re¿ ecting the wind-pollinated reproductive mode of the two species. The betweenspecies

component was 10-fold higher, 0.235, indicating two separate groups. A phylogenetic

tree estimated from allele frequencies also supported the presence of these two groups. Despite

the two well-separated groups in the tree, it was not possible to assign all individuals to the

species to which they supposedly belonged. In total, 10% of the Q. robur individuals and 14%

of the Q. petraea individuals were assigned to the other species, suggesting a limited amount

of introgression between the two species. Key words: gene conser×ation, genetic differentiation,

isozyme, population structure, Quercus petraea, Quercus robur.

, , Quercus petraea, Quercus robur.