This article described the population structure of trembling aspen (Populus tremuloides Michx.) in Alberta, a dioecious tree with continuous and wide distribution, and with a primary mode of reproduction through suckering. We studied random amplified polymorphic DNA (RAPD) variation in 249 trees from eight natural populations. Trees within a population were a minimum of 200 m apart to decrease the risk of sampling ramets of a single ortet. Of a total of 28 amplified RAPD products (bands) from five random oligonucleotide primers, the frequencies at seven (25%) were heterogeneous across populations and the percentage of polymorphism averaged 90.2 per population. Estimates of Shannon's phenotypic diversity index ranged between 0.58 and 0.69 among populations, averaging 0.65. There were 246 multiband phenotypes among the 249 trees; three were each shared by two trees from different populations and the remaining 243 were unique. Thus, trees within populations probably were different clones. Analysis of molecular variance partitioned the RAPD variation into the among- and within-population components. The within-population component accounted for 97.4% of the variation and was significantly different from zero at the 2% level of probability. The among-population component, although accounting for only 2.6% of the variation, was significantly different from zero at the 1% level of probability. Pairwise tests for the homogeneity of the RAPD variance between populations suggested significant divergences among 18 of the 28 (64%) population pairs.