The occurrence of transgenic herbicide-resistant oilseed rape (Brassica napus) in ruderal (non-crop disturbed) areas has not been investigated previously in Canada. The primary objective of this study was to document their occurrence in two main ruderal areas (along railways and roads) in the province of Saskatchewan, where half of all oilseed rape is grown, and at the port of Vancouver, British Columbia on the west coast of Canada, where most oilseed rape destined for export is transported by rail. During the 2005 growing season, leaf samples of oilseed rape plants were collected at randomly-selected sites along railways and roads across Saskatchewan ecoregions and at Vancouver; infestation area, density, and plant height of oilseed rape were measured at each site. The presence of the glyphosate and glufosinate resistance traits was determined using test strips. The infestation area of oilseed rape, averaged across 155 sampled sites in the Saskatchewan survey, was markedly smaller in populations along railways than roads; in contrast, infestation area averaged across 54 sites in the Vancouver survey was greater for populations along railways than roads. In both surveys, mean plant density was greater for populations found along railways than roads. Two-thirds of oilseed rape plants sampled across Saskatchewan ecoregions and at Vancouver were transgenic, although the relative proportion of plants with the glyphosate or glufosinate resistance trait varied between surveys. Frequency of occurrence of transgenic plants in ruderal areas was similar to the proportion of the oilseed rape area planted with transgenic cultivars in the recent preceding years. A single transgenic B. rapa x B. napus hybrid was found along a road in Vancouver, confirming the relatively high probability of hybridization between these two Brassica species. With current control measures, transgenic oilseed rape populations may persist and spread in these ruderal areas.