Wheat streak mosaic (WSM) caused by WSM virus (WSMV) is a serious disease of wheat which cannot be easily controlled using cultural practices or chemicals. Tolerance to WSMV exists in hexaploid spring wheat ('Triticum aestivum ' L) germplasm. BW155 is an advanced spring bread wheat line with tolerance to WSMV. Canada Western Red Spring (CWRS) wheat cultivar, AC Elsa is moderately susceptible, while AC Barrie and Laura are highly susceptible to WSMV infection. Wsm1, a gene for resistance to WSMV has been transferred into winter wheat from 'Agropyron intermedium'. The line 7166 is resistant to WSMV and carries the Wsm1 resistance gene. Both resistance and tolerance to WSMV contribute to the host's good performance. The overall objectives of this thesis were: (i) to determine the mode of inheritance of WSMV tolerance in BW155, and (ii) to combine BW155-derived tolerance and Wsm1 resistance derived from 7166. (Abstract shortened by UMI.)