SUMMARY Potassium and 14C-labelled proline, aspartic acid, glutamic acid, and alanine were accumulated by Salmonella oranienburg, during glucose oxidation, over a range of water activity (a,) values. In the absence of amino acids, potassium accumulation increased to a maximum as a, was decreased to about 0.975 but then dropped to a low value at 0.960 a,; with an amino acid, potassium accumulation was much increased and maximum uptake occurred at 0.960 a,. (14C)Proline uptake increased linearly with decrease in a,, the accumulated proline being metabolized in part to glutamic acid and to deaminated compounds; at 0.95 a, uptake of (14C)- proline reached 1-4 mmol/g dry bacteria. Uptake of aspartate was comparable to that of proline at aw down to 0.98 but then decreased. Relatively little exo- genously supplied glutamate or alanine was accumulated at any water activity. At a, levels below 0.98, oxygen uptake by non-growing bacterial suspensions in- creased with time and accompanied the linear uptake of proline. Proline accumula- tion ceased when a maximum rate of oxygen consumption was reached. At 0.97 a, the rate of proline uptake was unaffected by a mixture of 19 amino acids, but the total proline uptake decreased to one-half. In contrast, the proline homologue azetidine-2-carboxylic acid lowered the rate and extent of proline accumulation by one-third.