Chloroplasts isolated from the siphonous green alga Caulerpa simpliciuscula (Turner) C.Ag. were shown to be resistant to dissolution by the nonionic detergent Teric-10 at concentrations as high as 0.3% (v/v) when treated at 0 C. There was little release of stromal enzymes under these conditions. These chloroplasts were disrupted by osmotic shock as shown by measurement of the release of both glucose-6-phosphate dehydrogenase and NADP-dependent glutamate dehydrogenase into the suspending medium. Ribulose-1,5-bisphosphate carboxylase, an accepted marker for chloroplast stromal protein in higher plants, was largely retained in the disrupted chloroplast following the osmotic shock. This is considered to be due to the location of a significant proportion of enzyme within the pyrenoid, which protects it from dissolution and causes it to behave as though it were an insoluble protein.