In addition to the usual cell inclusions, Cyano phyceae possess a variety of inclusions of spo radic distribution which can be placed in seven groups of systematic value (Jensen 1984). Most observations were made on culture isolates. This note provides data on some unusual inclusions in cells of Lyngbya. Examination of natural material of Lyngbya subtilis W. West (Syn. L. limnetica Lemm., L. lacustris Lemm., Komarek pers. comm.) re vealed the presence of large, previously unde scribed, crystalline inclusions. L. subtilis is a fil amentous planktonic blue-green alga which occurs regularly in the eutrophic Plussee (lake near Pion, West Germany). Rectangular (Figs 13) and hexagonal (Figs 4-10) intracytoplasmic crystals were frequently found in filaments col lected at the end of the vegetative period. The crystals were investigated using light and trans mission electron microscopy and compared with other cyanophycean crystalline inclusions. The material for the present investigation was collected in October 1981. One portion of a plankton sample was fixed with 2% formalin. Subsamples from this formalin-fixed portion were treated with different dyes (neutral red, toluidine blue, coomasie blue, methylene blue, alcian blue, alkaline or acid fuchsine, Lugol's solutions and methyl violet) before being observed in the light microscope. Another part of the same formalin fixed portion was fiat embedded in Spurr's me dium according to the method later described by Reymond & Pickett-Heaps ( 1983). The ultrathin sections of hexagonal crystals (rectangular ones were too rare) were used for qualitative analysis. Some sections were investigated with X-ray mi cro-analysis (Erasmus 1978; Pueschel & Par thasarathy 1984) with an EDAX-microprobe mounted on a Philips 400 EM microscope, while other sections were used for enzymatic digestion tests. Pronase, trypsine, pep sine, papaine and collagenase were used. Two enzyme mixtures were similarly used: the first comprised cellulase, pectolyase and hemicellulase; the second, amy loglucidase, alphaand beta-amylase. The tests were repeatedly carried out on gold slot grids (copper and nickel grids were unsuitable for tech nical reasons) immersed in 0.2-2% enzyme so lutions for 24 h at 37°C (Monneron 1966; Mon neron & Bernhard 1966; Horobin 1982; Thorsch & Esau 1983). Adjacent control sections were identically treated in the absence of enzymes. All grids were stained with uranyl acetate and lead citrate. Another portion of the material, fixed with 0.5% glutaraldehyde and 1 % osmium te troxide (Pickett-Heaps et al 1978), was used for routine ultrastructural studies. Rectangular (0.5 !Lm long, 0.4 !Lm broad) and hexagonal (up to 1.5 !Lm long, 1 !Lm broad) in tracytoplasmic inclusions were occasionally ob served, mostly in old filaments characterized by large portions of empty sheaths with numerous, attached bacteria. The rectangular crystals were rare (Figs 1-3), while the hexagonal crystals were more abundant. One to five crystals (Figs 4-10) were irregularly distributed in one L. subtilis fil ament, each occupying up to two thirds of the cell volume. The inclusions were always located close to the transverse cell walls. There was no evidence of a boundary membrane, and both types of inclusions had a crystalline sub-struc ture. The rectangular inclusions had two series of electron dense lines crossing at right angles with spacings of about 8.5 nm and 13 nm (Fig. 3). The hexagonal crystals had three series of elec-