Yan, W., Pageau, D., Frégeau-Reid, J., Lajeunesse, J., Goulet, J., Durand, J. and Marois, D. 2011. Oat mega-environments and test-locations in Quebec. Can. J. Plant Sci. 91: 643-649. The Quebec agricultural regions have traditionally been divided into three zones. Zone 1 includes the small, southern most regions in the Montreal plain, zone 3 includes the large, discontinuous northern regions spreading from the west to the east of the province, and zone 2 includes areas between zone 1 and zone 3. Genotypic main effect (G) plus genotype-by-environment interaction (GGE) biplot analysis of the yield data from the Quebec Oat Registration and Recommendation trials during 2006-2009 revealed that the Quebec oat-growing regions can be divided into two distinct mega-environments: the small zone 1 region and the much larger zone 2 plus zone 3 regions. Due to the large genotype by mega-environment interactions, cultivar evaluation and recommendation must be conducted specifically to each mega-environment. However, a zone 3 test location, La Pocatière, consistently behaved like a zone 1 location in terms of cultivar ranking. Therefore, La Pocatière cannot be used to represent the zone 3 region, and cultivar recommendation for the regions represented by this location should be based on data from this location plus the zone 1 locations. In addition, climatic and soil data were examined in an attempt to explain this observation. The methodology adopted in this work may be of value to similar studies for other crops and in other regions.