The discovery in southwestern British Columbia of widespread established populations of the Palaearctic leafminer Heterarthrus vagans (Fallén) feeding on native red alder, Alnus rubra Bong. (Betulaceae), is reported. A preliminary survey suggests that it is currently confined to the Fraser Valley west of Hope, its distribution extending north along Howe Sound almost to Squamish. It was not found in the greater Victoria area. No other regions of the province have been surveyed. This introduced sawfly completes at least two generations a year. Diagnostic characters to aid the recognition of adults and feeding larvae of H. vagans and a modified key to adult Heterarthrinae on Betulaceae are provided.