The effect of NaC1 on the growth rates and yields of 31 gram-negative, heterotrophie, marine bacteria was determined. The strains used were representative of aerobic genera (Alteromonas, Pseudoraonas, Alcaligenes, Bdellovibrio) as well as genera comprised of facultative anaerobes (Beneckea, Photobacterium). Two media were used--the first, a medium designed for the cultivation of marine bacteria and, the second, a medium used for the cultivation of terrestrial strains. These two media differed in the concentrations of divalent cations; the terrestrial medium (TM) conrained 2 mM Mg ++ and 0.55 mM Ca ++ while the marine medium (SfM) contained 50 InN Mg ++ and 10 mM Ca ++. The amount of NaC1 necessary for optimal grownbh varied in different strains and was usually considerably higher in TM (100 to 460 mM) than in MM (70 to 300 mM). Many strains which grew in MM and TM had a shorter generation time in the former than in the latter medium. In addition, four strains which grew well in MM usually failed to grow in TM. These results show that higher levels of divalent cations are either essential for growth or stimulate growth rate, indicating that for many marine strains a terrestrial medium modified by the addition of NaC1 cannot support optimal growth. Fourteen terrestrial strains of the genera Pseudomonaz, Alcaligenes, Acinetobacter, Salmonella, Aeromonas, and Vibrio did not have ionic requirements comparable to those of the marine strains. All of the terrestrial organisms grew in TM without added NaC1 (0.068 mM Na + was present as a contaminant). In some terrestrial organisms, growth was stimulated by the addition of NaOl, the highest stimulation being found in Vibrio choleras. The optimal growth rates and fields for four strains of this species were observed at 2.5 to 5.0 mM NaOl while the gTowth rates and yields in TM with no added NaOl were 40 to 50% of the optimum.