Uptake of several naturally occurring organic solutes by the unicellular cyanobacterium Microcystis sp. caused changes in cell turgor pressure (p(t)), which was determined by measuring the mean critical pressure (p(c)) of gas vesicles in the cells. Cells had an initial p(t) of 0.34 MPa, which decreased to 0.08 MPa in 0.15 M sucrose. In solutions of polyols, p(t) gradually recovered as the solutes penetrated the cytoplasmic membranes. From measurements of the exponential rate of turgor increase, cell volume and surface area, the permeability coefficient of the cytoplasmic membrane to each solute was calculated. Permeabilities to amino acids, ammonium ions and sodium acetate indicated little passive movement of these substances across the cell surface from solutions at high concentrations. We looked for evidence of ion trapping of acetic acid: at low pH there was a rapid rise in turgor pressure indicating a rapid uptake of this weak acid. After 20 min the turgor was lost, apparently due to loss of integrity of the cell membranes. For cells in natural habitats, studies of the permeability of cells to solutes is relevant to the problem of retaining substances that are accumulated by active uptake from solutions of low concentrations in natural waters.