In 1987, leaf rust of wheat caused by Puccinia recondita f. sp. tritici was first observed in the Red River Valley of Manitoba in the first week of June. By early August the disease was widespread throughout the eastern prairies, although yield loss was minimal as grain filling occured before infections became heavy. Fifty-five virulence combinations of P. recondita f. sp. Tritici were detected in 1987 in Canada using 15 lines of Triticum aestivum cv. Thatcher, each with a single gene for leaf rust resistance, as differentials. Virulence was not detected to resistance genes Lr 19, Z>25, and Lr29. The incidence of isolates and virulence on LrlA increased dramatically from previous years, while virulence to Lrld was detected at low levels. Virulence to Lr 16 remained low. The leaf rust population in Manitoba and Saskatchewan was dominated.