b-galactosidase production by Streptococcus salivarius subsp. thermophilus increased as the whey concentration raised up to 5.48% (4.0% lactose).  Lactose was exhausted till 4.11% whey concentration (3.0% lactose) after 24 hours.  Growth rates and lactose utilization were increased as the whey concentrations increased up to 5.48% whey (4.0% lactose).  Lactose media yielded 42.93% less b-galactosidase activity than whey media.  A 21.33% decrease in lactose utilization was observed while about 5.0% increase in growth rate was obtained.  The decrease of pH was much smaller in the whey- than in lactose-containing medium.  Maximum b-galactosidase production by bac erial species in whey medium was observed  between pH 6.5 to 7.5.  Whereas maximum growth rate recorded at pH 6.0 to 7.5.  Cells lowered the pH from an initial pH of 7.0 to 4.5 after 24 h of growth.  A progressive increase in b-galactosidase production by Str. salivarius subsp. thermophilus  was observed up to 24 h of incubation, after which it become constant for another 24 h, whereas growth rate was still increased.  The optimum temperature for b-galactosidase production coincided with the optimum temperature for the   growth of the organism, about 40^oC.  Lactose utilization by the organism also increased around the optimum temperature.  Higher b-galactosidase levels from Str. salivarius subsp. thermophilus were obtained when either whey (4.0% lactose) or whey (2.0% lactose) supplemented with 2.0% galactose was used.     Supplementation of whey with   glucose repressed b-galactosidase formation.  Supplementation of whey with other sugars tended to decrease the level of b-galactosidase to variable degrees.  (NH₄)₂SO₄ promoted the highest level of b-galactosidase biosynthesis in Str.  salivarius subsp. thermophilus than other nitrogen sources. The stimulation in b-galactosidase production by organic nitrogen supplements.  In general, nitrogen sources did not have great effect on the growth rate or lactose utilization. b-galactosidase production increased with increasing of (NH₄)₂SO₄ concentration until reaching a maximum yield for enzyme at 0.5%.  Further increase (NH₄)₂SO₄ concentration has diverse effect on increasing enzyme production and growth rate.  NaH₂PO₄ supported the highest level of b-galactosidase production by tested bacterial species.  Whereas the other phosphorus sources tended to decrease the level of enzyme to variable degrees.  The maximal b-galactosidase biosynthesis was reached at a 0.5% (w/v) level of   NaH₂PO₄.  The supplementation of whey with Mn⁺⁺, Mg⁺⁺, Ca⁺⁺ and Na⁺ led to significant stimulation for bacterial b-galactosidase production.  Monovalent salts, i.e., K⁺ or Na⁺ exerted a slight stimulatory effect indicating that the levels of these salts in whey were sufficient.