Eighteen (18) provenances and 199 families of Jug/ans regia were evaluated in three provenance trials and one provenance / progeny trial in southern England. Variation among and within provenances and narrow-sense heritability were estimated for growth characteristics (five-year and seven-year height, seven-year collar diameter), flushing date, spring f!ost injury and juglone content. Variation within provenances was much greater than variation among provenances (45.6 % and 41.2 % of the total variation for seven-year height and collar diameter respectively). Variation due to provenance x site interactions accounted for less than 3 % of the total variation. Narrow-sense heritability across provenances was low for growth characteristics (0.02 for five-year height, 0.07 and 0.06 for seven-year height and collar diameter respectively), but high for flushing date and spring frost injury (0.50 and 1.01 respectively). The genetic correlation between five-year and seven-year height was very high (1.00). Genetic correlations between flushing date and five and seven-year heights were 0.68 and 0.25 respectively. There was a significant positive correlation between flushing date and latitude of provenances, and highly significant negative correlations between flushing date and frost injury. Overall mean juglone content in leaves was 357.0 mg / 100 g dry weight. Narrowsense heritability for juglone content was extremely low (0.01). Correlations between juglone content and growth traits were negative and highly significant. Genetic variation among ten natural populations of Jug/ans regia from Kyrgyzstan and Turkey was studied using six microsatellite loci. Average observed and expected heterozygosity and number of alleles per locus were 0.54, 0.63, and 4.4 respectively. Pairwise F51 values showed that populations from Turkey were significantly different from populations from Kyrgyzstan and also from each other. For microsatellite loci, variation among populations accounted for more than 40% of the total variation, while for quantitative traits among-population variation never exceeded 12% of the total.