Forests can be dissected or internally fragmented by anthropogenic linear clearings. Much research has focused on roads but in forests overlying oil and gas reserves, seismic lines (narrow exploration trails) also internally fragment forests and alter landscape structure. Seismic lines are of particular interest because they already exist in western North America and exploitation of future reserves may require new seismic line clearing over vast forest areas. An assessment was needed to compare their relative contribution to forest fragmentation against other more well-known linear forest clearings. This study was conducted across an area of 4022 km² of boreal forest in western Canada. Seismic lines directly occupied a relatively small area (1% of all land), but were five times longer than roads and rail lines. Seismic line density was more than twice that of roads, rail lines, power lines and pipelines combined and accounted for 80% of all edges. Seismic lines have the potential to indirectly influence more forest than all these other types of linear forest clearings. Seismic lines consistently decreased the size of forest patches, and increased the number of patches across spatial scales from 5.0–4900 ha but tended to have a greater impact at larger spatial extents. While roads are the most important agents of fragmentation in some forests, in forests where oil and gas reserves are exploited, seismic lines have the greatest impact on forest fragmentation.