1 542 257 strains of Salmonellae isolated in 109 countries during the period 1934-75 were analysed to determine the relative frequency of the 1864 species currently recognized. In addition to the absolute numbers of strains isolated, the "age" of the species and the numbers of reports were used to compute a frequency quotient (QF). It is proposed, on the basis of occurrence, that species be classified as most frequent, frequent, uncommon, rare and very rare. The Salmonella species most frequently isolated from man, food animals, other animals, foods, feeding stuffs, and water including sewage are listed in tables.