title: Erratum.
abstract: In the report "Iron photoreduction and oxidation in an acidic mountain stream" by D. M. McKnight et al. (29 Apr., p. 637), reference 17 [E. L. Madsen, M. D. Morgan, R. E. Good, Limnol. Oceanogr. 31, 382 (1986)] was cited ( p. 638) as indicating "that biological processes are not responsible for light-induced Fe(II) production." This possibility was not addressed by Madsen et al., nor did Madsen et al. use both poisoned and natural streamwater-sediment mixtures to distinguish between biotic and abiotic contributions to iron reduction. In table 1 of the same report, the values in columns two and three were incorrect. They should have been, for sodium, calcium and magnesium, respectively, 2.7 +/- 0.34 mg liter(-1)', 13.0 +/- 0.73 mg liter(-1), and 4.3 +/- 0.24 mg liter(-1).