Summary1Methods of establishing pure cultures of rusts are discussed and a new spore‐proof and insect‐proof cellophane cylinder is described.2Eighty‐two cultures, isolated from collections ofP. anomalain Great Britain, Portugal and Spain, have been tested for the determination of physiologic races. The barleys used as differential hosts were those used by Hey and Mains and also Aegyptische 4‐zeilige Sommergerste. Eleven new physiologic races have been identified and are numbered 12–22. Race 12 was found to be widely distributed in Great Britain, Portugal and Spain, the other ten were localized. Races 13, 14, 15 and probably 18 were found in Britain, races 18, 19 and 20 in Portugal, and races 16, 17, 21 and 22 in Spain.3A mutant, differing in colour (orange and yellow) and in pathogenicity, is recorded forP. anomala.The mutant arose from an unstable culture of race 14 cultivated onHordeum vulgare pallidum(Sudan), and is numbered physiologic race 23.4Segregation and hybridization of physiologic races occurred through the aecidial stage onOrnithogalum umbellatumL. The heterozygous condition of physiologic races 12, 13 and 23 was demonstrated. Self‐fertilized material from race 12 gave rise to the parent race, to races 16 (?) and 19 and to four new races described under the numbers 24, 25, 26 and 27. From self‐fertilized material of race 13, races 12(?), 13, 22 and 24 were isolated and also two new races numbered 28 and 29. From self‐fertilized material of race 23, races 12, 19, 20 and 22 were isolated. A reciprocal cross between physiologic races 13 and 23 was achieved; in one direction the progeny gave rise to a single physiologic race, which proved to be new and is numbered 30; in the other direction the two physiologic races 30 and 18 were produced. This result is discussed and is attributed to cytoplasmic inheritance.5Several biotypes of physiologic races 12 and 13 were found; they seem to establish a linkage between the two races.6A new selection of differential hosts for the differentiation of physiologic races ofP. anomalaon cultivated barleys is proposed. A “Numerical Table” of the thirty physiologic races and an “Analytical Key” for their determination are given.7The distribution of physiologic races ofP. anomalaon cultivated barleys in Great Britain, Portugal and Spain is discussed.8From field observations it is concluded that at CambridgeP. anomalamay overwinter in its uredospore stage. Evidence is given that in Portugal the uredospores survive the