Colonies of the prymnesiophyte marine microalga Phaeocystis globosa were tested for mechanical properties, permeability and biochemical composition using the micropipette aspiration technique. We found that the Phaeocystis colony is enclosed by a thin, yet very strong, semi-permeable skin (pore size between 1 - 4.4 nm diameter) with plastic and to a limited extent also elastic properties. Qualitative staining of single colonies with selective fluorescent dyes indicated absence of lipophilic compounds and chitin but presence of aminogroups in the colony skin. Individual cells in the colony appear to be weakly connected with one another and attached to a very dilute, peripheral gel. Suction applied to the colony resulted in volume loss due to expulsion of water and squeezing together of the cells within the skin into a tight pouch; the presence of any firm gelatinous matter within the colony was not discernible. On increasing suction pressure, the skin eventually ruptured and the cells were sucked out of the hole leaving the empty skin behind. We propose that the skin effectively protects the colony cells from grazing and infection by viruses and other pathogens. The unsuspected presence of a skin is probably the main reason why Phaeocystis colonies have reduced mortality relative to solitary cells and form large blooms in many regions of the world ocean. Our findings indicate that the colonies should be viewed as "bags of water" rather than "balls of jelly".