At a constant soil temperature of 18°C, ‘Abe’ wheat was resistant to Biotype B of the Hessian fly,Mayetiola destructor(Say). However, when Abe was exposed to 1-day-old Biotype B larvae at 27°C for one day and then transferred to 18°C, more than ½ of the plants were susceptible. Also, each additional day of exposure to 27°C caused susceptibility to increase until it reached 100% after 7 days. In the reciprocal test, with one day of exposure to 18°C, only 4% of the plants were resistant, but plants exposed for 4 days to 18°C were 100% resistant and not affected by ensuing exposure to 27°C.