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Large numbers of a brown to violet ciliate can often be seen in freshwater lakes and billabongs in Australia. Uninterpreted records by light microscopy and electron microscopy are provided. The ciliates are identified as Stentor amethystinus Leidy, 1880. Despite the abundance of this species, this is a new record for Australia. Heep, Thomas, Jan Rohozinski, Alastair Simpson & David J. Patterson, 1998. Stentor amethystinus (Protista: Ciliophora: Heterotrichida), a common protozoan member of fresh-water plankton in Australia. Records of the Australian Museum 50(2): 211-216. Stentor Oken, 1815 (Oken, 1815) is a widespread and familiar genus of heterotrich ciliates (protozoa). Cells are typically trumpet-shaped and often distinctly coloured; different species being green, black, blue, pink or brown (Foissner & Wolfl, 1994; Kahl, 1932; Tartar, 1961). A brown species of Stentor has been reported as occurring in very large numbers in fresh-water bodies in New South Wales, the Australian Capital Territory and Victoria (Laybourn-Parry et al., 1997). It occurs throughout the year but blooms, which can discolour the water, occur at various times from mid-spring to late autumn. Up to 4200 individuals per litre have been counted, and the organism is argued to make a significant contribution to primary production (Laybourn-Parry et al., 1997). It has also probably been reported as "cf. Climacostomum" (Walker & Hillman, 1977). The aim of this study was to establish the identity of this ciliate. The genus Stentor has been reviewed most recently by Foissner & Wolfl (1994) who recognised 19 species. Species distinctions are normally made on the basis of cell shape and size, presence/absence of a mucilagenous sheath, macronuclear form, the number of micronuclei and their location, numbers of kineties and membranelles, pigmentation, presence/absence of symbiotic algae. One study (Nilsson, 1986) has added ultrastructural characteristics to assist in distinguishing species. Brown species 212 Records of the Australian Museum (1998) Vol. 50 have been observed from America (Leidy, 1880), India (Murthy & Bai, 1974), Africa (Nilsson, 1986), South America (Foissner & Wolfl, 1994; Laybourn-Parry et al., 1997), New Zealand (cited in Laybourn-Parry et al., 1997) and from Europe (Foissner et al., 1992; Schneider, 1992). The brown colour results from the combination of symbiotic green algae and numerous dark plum-coloured pigment granules. The first description of Stentor in Australia was of S. polymorphus (Shephard, 1891), Shephard found large numbers in pools in the area of Sandringham, Victoria. Other records of S. polymorphus in Australia are provided by Schewiakoff (1893) and Gillies (1915). Other species reported from Australia are: S. igneus (Shephard, 1904), S. roeselii (Stickland & Stickland, 1895), and several unnamed species (Fielder, 1893; Ingram et al., 1997; Laybourn-Parry et al., 1997; Stickland, 1923). Materials and methods Stentor was collected in 1994 from Corin Dam and Lake Burley Griffin, both in the Australian Capital Territory close to Canberra, Googong Dam in New South Wales, in 1996, from Lake Tuggeranong (ACT, Canberra) and from a pond near Camden, south-west of Sydney. Water samples containing Stentor were kept at 5 to 14°C in plastic bottles. Attempts to establish long term cultures of the organism have been unsuccessful. Exposure to temperatures above 23° results in a rapid decrease in the number of individuals. Stocks were maintained for up to three months at 18°C under continuous light. Live ciliates were used for morphological studies. For some detailed studies, a small drop of O.OlM EGTA and O.lM MgCl2 was added on a slide to stop movement and prevent contraction. Movement and morphology were documented by video records and black and white photography using a Leica DM microscope equipped with a dedicated flash system (Patterson, 1982) The sizes of cells were measured using an ocular micrometer. To obtain typical cell profiles, groups of about 100 cells were left undisturbed on a dissection microscope for 12 or more hours in the dark. For electron microscopy, cells were fixed simultaneously with ice cold 2.5% glutaraldehyde and 0.2% osmium tetroxide in 50 mM cacodylate buffer. Cells were washed free of fixative in buffer, collected, and dehydrated through a series of ethanols before embedding in Araldite. Fixed material was sectioned with a diamond knife using a Reichert Ultracut S ultramicrotome. Sections were collected on Pioloform coated slot grids, carbon coated, and stained with lead citrate and uranyl acetate prior examination using a Zeiss 902 electron microscope.