Sixty-three wild strains of Proteus mirabilis were investigated. All 55 strains comprising groups la and 1b were found to be cryptic with regard to sucrose fermentation; they possessed competent enzyme systems but did not normally ferment the sugar. No enzyme capable of cleaving this sugar was extracted from the 3 strains of group 2 and the 5 strains belonging to group 3. Partially purified enzyme preparations from two strains of P. vulgaris and two cryptic P. mirabilis strains were investigated; all four were constitutive [beta]-D-fructofuranosidases capable of splitting raffinose to melibiose and fructose. Sucrose uptake studies showed that strains of groups la, lb and 2 did not accumulate sucrose from 1% (w/v) solution; the strains of group 3 accumulated large amounts of sucrose. None of the P. mirabilis strains were permeable to maltose. The permeability barrier for sucrose was overcome by increasing its concentration to 5% (w/v). Under these conditions groups la and lb strains fermented sucrose in peptone water within 36 hr. Sodium deoxycholate also changed the permeability barrier of some of the cryptic strains, enabling them to ferment 1% (w/v) sucrose promptly. After 3-11 days in 1% (w/v) sucrose peptone water all 55 cryptic P. mirabilis as well as the strains of group 3 fermented sucrose. This fermentation was not caused by wild-type organisms but resulted from the selection of sucrose-positive mutants which arose from the former and were capable of prompt sucrose fermentation. The mutants of strains of group 3 arose at lower rates than those from the cryptic strains. It is concluded that selective permeability to sucrose and [beta]-D-fructofuranosidase activity are genetically distinct properties of Proteus. A scheme for the classification of phenotypes of P. hauseri is presented.