When British isolates of Ceratocystis ulmi were surveyed for compatibility type, both A- and B-types were found in the non-aggressive strain, but only the B-type in the aggressive strain. Single ascospore progeny from crosses between compatible aggressive and non-aggressive isolates showed a near-normal growth rate distribution, with a mean lying between the parents. Many grew either faster than the aggressive or slower than the non-aggressive parent. The progeny were highly variable in culture morphology and could not be classified in terms of the parental types. When inoculated into English elm they showed a marked skewness towards low pathogenicity. None approached the aggressive strain in pathogenicity. It is concluded that the above characters are under polygenic control, and that the aggressive strain could not arise from the non-aggressive by a simple mutation. The results suggest that the two strains may be reproductively isolated.