A survey was made of the NaCl tolerance of approximately 1,300 Streptomyces strains belonging to 313 species. The growth medium of the organisms was supplemented with a graded series of NaCl concentrations (4, 7, 10, and 13%). Only 1.8% of the species could not tolerate 4% NaCl; 26.9% could grow at a maximum of 4%; 49.7% could tolerate a maximum of 7%; 18.8% could grow at a maximum of 10%; and only 2.8% could tolerate 13% NaCl. In evaluating the relationships of NaCl tolerance to various taxonomic features, higher tolerance was statistically associated with the "yellow" and possibly the "white"-spored streptomycetes, whereas the "red"-spored series tended to have lesser tolerance. Higher tolerance was also indicated for spiny-spored species, as a group, than for smooth-spored forms. Likewise, nonproducers of melanin, collectively, were more NaCl tolerant than melanin-producing species. Uniformity of test responses between strains of species studied suggested the usefulness of NaCl tolerance as a taxonomic criterion.

Streptomycetes comprising the Streptomyces hygroscopicus-like complex may be divided into two groups based on the micromorphology of their spores. The species S. hygroscopicus has short-cylindrical, phalangiform spores, whereas members of the second group possess elliptical-type spores. It was determined that S. platensis, of the latter group, is not only readily distinguishable from S. hygroscopicus on the basis of spore micromorphology, but differs, as well, according to various physiological measurements.