SUMMARYPathogenicity tests with forty‐nine isolates ofPhytophthora fragariae, cause of red core disease of strawberry, using the strawberry varieties Huxley, Perle de Prague, Climax, Talisman, Aberdeen and Cambridge Vigour, revealed the existence of twelve physiological races of the fungus on the basis of a pathogenic or non‐pathogenic reaction. Further differentiation, on the basis of degree of pathogenicity to Perle de Prague and to Climax, allowed the recognition of nineteen races.Varying the concentration of zoospores used as inoculum, and the length of inoculation period, which did not affect the relative pathogenicities of two isolates tested, had greater effects on the level of invasion with a highly pathogenic isolate than with a slightly pathogenic one.Amongst the isolates differences uncorrelated with other characters such as pathogenicity were demonstrated in growth characters in agar culture and in abundance of zoospore product