Summary.Physiological characteristics of 133 strains of coryneform bacteria originating from milk, milk products and dairy equipment were studied. Based on the results of a number of selected tests, the majority of these strains have been classified in four groups, the first three comprising organisms derived from aseptically drawn milk. Group I strains were lipolytic, nonhaemolytic, nonproteolytic, V‐P and urease positive, grew in the presence of 9 NaCl, and had a nutritional requirement fulfilled by Tween 80. They were identified asCorynebacterium bovis.Group II strains were also urease and V‐P positive, but were nonlipolytic, haemolytic, salt sensitive, and they liquefied gelatin slowly at 22°. Marked changes in morphology were shown; cells from 6–8 h cultures were predominantly large coryneform rods, whereas those from 24–36 h (or older) cultures were almost uniformly coccoid. Evidence is presented in support of the identification of these strains asC. ulcerans.Group III comprised catalase and V‐P negative, lipolytic organisms which liquefied coagulated serum; they grew optimally on media containing blood in a CO2‐enriched atmosphere and were identified asC. pyogenes.The last group consisted of organisms isolated from market milk, milk products and dairy utensils; presumably for these the udder is not a suitable habitat. They were salt sensitive, V‐P negative, nonhaemolytic and nonlipolytic, and, although exhibiting variable heat resistance, closely resembledC. lacticum.Although heat labile, some of the strains which had pronounced caseolytic activity were otherwise similar to descriptions ofC. liquefaciens; however it is held that there are insufficient grounds to justify their taxonomic separation fromC. lacticum.A residue of 11 isolates originating from the same sources as Group IV strains could not be satisfactorily classified and have been placed in a Misce
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