Plants of Lolium perenne L., Holcus lanatus L., Lotus corniculatus L. and Anthoxanthum odoratum L. were exposed to fog treatments at pH values of 2.5, 3.5, 4.5 and 5.6. There were three 4-h exposures per week providing a total of 6 mm deposition. Supplementary watering with pH 4.5 simulated acid rain provided a further 24 mm deposition per week. Plants of Lotus corniculatus showed reduced growth at pH 2.5 and 3.5 compared with the less acidic fog treatments and greatest dry matter accumulation at pH 4.5. In contrast, plants of the other three species showed greater dry weights in the most acidic pH 2.5 treatment in comparison with other treatments. For H. lanatus and A. odoratum, the increased dry matter production at pH 2.5 was associated with a reduced root/shoot ratio. There was a promotion of flowering stem production in the pH 2.5 treatment for Lolium perenne and increased numbers of flowers and buds in the pH 5.6 treatment for Lotus corniculatus. It is suggested that leguminous species are more susceptible to acid deposition than other plant families.