SUMMARYThe wheat cultivar Hybride de Bersee is described as possessing durable resistance to yellow rust. Some races ofPuccinia striiformiscan infect it at the seedling stage but not severely in the field. Using euploid and aneuploid stocks of Bersee it was shown that a large part of this resistance was controlled by chromosome 5BS‐7BS, the effect being detectable in seedlings and at later growth stages. Measurements of spore production from infected seedlings showed that the line ditelosomic for the 5BS arm of the chromosome was more resistant than the line ditelosomic for the 7BS arm under environments favouring high spore production, but more susceptible under environments favouring low spore production. Thus both arms of the chromosome could contribute to resistance in appropriate environments. Chromosome 5BS‐7BS did not carry the factors controlling the race‐specific resistance to race 37 E132 ofP. striiformis.The possibility of exploiting the resistance carried by chromosome 5BS‐7BS in wheat breeding programmes is di