Marine and sea-run (anadromous) Threespine Stickleback (Gasterosteus aculeatus) have repeatedly colonized and adapted to freshwater habitats since the last glacial retreat. A wide range of body sizes has evolved in freshwater populations. There have been numerous studies of the genetics of convergent evolution for morphological, physiological, and behavioral traits in Threespine Stickleback, but there are few studies of the genetic architecture of body size or growth rate in natural populations of any species. The stickleback system provides a unique opportunity to study size divergence using “replicate” natural populations. We collected samples near the expected extremes of body size among Threespine Stickleback populations from 28 lakes in the Matanuska-Susitna Valley and on the Kenai Peninsula, Alaska. We measured the body length and weight of males and females from one putatively large-bodied and four putatively small-bodied stickleback populations. We compiled existing data from previously measured samples and those that we measured and classified 11 populations as large, small, or average based on their size distributions. We found that females were typically longer than males, except for Jack Lake where there was no apparent sexual dimorphism for body size. Based on these classifications, appropriate populations were chosen for a genotype-phenotype association analysis. Several candidate growth genes, including Growth Hormone and Insulinlike Growth Factor, will be analyzed to see which are associated with growth differences within and among populations.