Anthropogenic inputs of selenium to natural waters have increased this century due to release of the element during coal and oil combustion, and its discharge in wastes (Nriagu, 1989). Selenium is of interest as it is classified as an essential element for animals (Schwartz & Faltz, 1957) but is toxic at elevated levels (Ganther, 1974; Moxon & Olson, 1974). Sublethal effects, e.g. oedema, tissue degeneration and chromosomal aberra- tions, are also prevalent in animals exposed to high levels of selenium from natural or anthropogenic sources (Sorensen &Baver, 1983; Sorensen et al., 1984; Gillespie & Baumann, 1986). Recent studies indicate that selenium is probably incorporated into enzymes as selenocysteine, and because of the existence of genetically coded codons, may be considered as the 21st amino acid (Reches et al., 1994).