Deep-sea Cibicidoides pachyderma (forma mundulus) and related Cibicidoides spp. were cultured at in situ pressure for 1-2 d, or 6 weeks to 3 months. During that period, fluorescence analyses following BCECF-AM (2.sup.',7.sup.' -bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester) or calcein (bis[N,N-bis(carboxymethyl)aminomethyl]-fluorescein) labelling revealed a persisting cytoplasmic sheet or envelope surrounding the Cibicidoides tests. Thus, the Cibicidoides shell can be considered as an internal rather than an external cell structure. A couple of days to a week after being transferred into high-pressure aquaria and adjusted to a pressure of 115 bar, the foraminifera changed from a mobile to a more or less sessile living mode. During this quasi-sessile way of life, a series of comparably thick static ectoplasmic structures developed that were not resorbed or remodelled but, except for occasional further growth, remained unchanged throughout the experiments. Three different types of these permanent structures were observed. (a) Ectoplasmic "roots" were common in adult C. pachyderma, C. lobatulus, and C. wuellerstorfi specimens. In our experiments single ectoplasmic roots grew to a maximum of 700 times the individuals' shell diameter and were presumably used to anchor the specimen in an environment with strong currents. (b) Ectoplasmic "trees" describe rigid ectoplasmic structures directed into the aquarium's water body and were used by the foraminifera to climb up and down these ectoplasmic structures. Ectoplasmic trees have so far only been observed in C. pachyderma and enabled the tree-forming foraminifera to elevate itself above ground. (c) Ectoplasmic "twigs" were used to guide and hold the more delicate pseudopodial network when distributed into prevailing currents and were, in our experiments, also only developed in C. pachyderma specimens. Relocation of a specimen usually required it to tear apart and leave behind the rigid ectoplasmic structures and eventually also the envelope surrounding the test. Apparently, these rigid structures could not be resorbed or reused.