Notes: The ecophysiology of the hypotonic response was studied in the charophyte alga, Lamprothamnium papulosum, which was grown in a marine (SW; 1072 mosmol kg–1) and a brackish (1/2 SW; 536 mosmol kg–1) environment. The cells produced an extracellular mucilage identified by histochemical staining as a mixture of sulphated and carboxylated polysaccharides. The thickness and chemical composition of the mucilage layer was a function of environmental salinity and cell age. Mucilage progressively increased in thickness from the apex (9 SW cells: 12·6 ± 1·8 μm; 15 1/2 SW cells: 4·8 ± 0·7 μm) to the base of the plants (15 SW cells: 44·8 ± 3·3 μm; nine 1/2 SW cells: 23·8 ± 2·5 μm); with a corresponding increase in the sulphated proportion. The mucilage was significantly thicker in SW plants. Hydraulic conductivity (Lp) at the apex of SW plants, measured by transcellular osmosis, was 8·3 × 10–13 m s–1 Pa–1. This was close to Lp of freshwater Chara (8·5 × 10–13 m s–1 Pa–1) which lacked mucilage. Basal SW cells with thicker mucilage had a smaller apparent Lp of 3·5 × 10–13 m s–1 Pa–1. The electrophysiology of the resting state and hypotonic response was compared in cells from the two environments based on current/voltage (I/V) analysis. The resting potential difference (PD) and conductance differed (11 SW cells: – 102·4 ± 10·1 mV, eight SW cells: 18·6 ± 2·4 S m–2; 19 1/2 SW cells: –125·7 ± 5·9 mV, 8·3 ± 0·8 S m–2). The type of cellular response to a hypotonic shock (decrease of 268 mosmol kg–1) also differed. In 1/2 SW plants, only the apical cells with thin mucilage responded classically with depolarization, conductance increase, Ca2+ influx, cessation of cytoplasmic streaming, and K+ and Cl– effluxes. Older cells making up the bulk of the plants responded with depolarization, but continued cytoplasmic streaming, and had only a small increase in conductance; or depolarized transiently without altering the I/V profile, conductance or streaming speed. Most cells remained depolarized and in the K+ state 1 h post-shock. Cells treated with the K+ channel blocker tetraethylammonium chloride also depolarized and remained depolarized. The SW cells depolarized but otherwise responded minimally to a 268 mosmol kg–1 drop in osmolarity and required a further 268 mosmol kg–1 down-step to elicit a change in the conductance. A spectrum of responses was measured in successively older and more mucilaginous cells from the same marine plant. We discuss the ecophysiological significance of the mucilage layer which modulates the cellular response to osmotic shock and which can be secreted to different degrees by plants inhabiting environments of different salinity.