Production of biochemicals from red algae will become an agro-industrial reality only after improvement of strain through genetic manipulation has been achieved. In the absence of sexual reproduction, preparation of protoplasts is a prerequisite for genetic improvement of new strains. Although preparation of protoplasts from plant cells is a common technique, its application in red algae was limited. The unicllular alga Porphyridium sp. is encapsulated in a sulfated polysaccharide, the structure of which is still not fully known. A crude extract of a dinoflagelate Gymnodinium, a natural predator of Porphyridium cells in open cultures, was found to degrade Porphyridium sp. polysaccharide enzymatically. Porphyridium cells treated with the crude Gymnodinium extract were exposed to various osmotic media (0-1·5 m sucrose), and their volume was measured. Volume increase was observed in diluted sucrose solutions up to 0·175 m. While further dilution of the external osmoticum to 1·0 m had little effect, dilution to 0·0 m (distilled water) led to cell rupture. Elevated concentrations of external osmoticum resulted in shrinkage of the treated cells. Such osmotic behavior indicates exposure of the cells and thus cleavage of the capsule. The treatment did not affect the viability of the cells, as evidenced by fluorescein diacetate (FDA) fluorescence, nor did it affect the respiration rate, but it did lower the photosynthetic rate to some extent. The growth curves for the treated cells exhibited a longer lag time than in the non-treated controls. Lowering the NaCl content in the growth medium resulted in a further increase in the lag time of the treated cells. These results indicate that the treatment lowers the ability of Porphyridium cells to divide. Ability to divide is eventually recovered with time, the recovery apparently depending upon the external osmoticum. The results indicate that Gymnodinium crude extract degrades Porphyridium cell wall and thus can be used for protoplast production.