The water fern Azolla pinnata R. Br. was fumigated for 1 week with either 25, 50 or 100 nl 1−1 SO2. The symbiosis of Azolla with Anabaena azollae (spp.) was severely damaged by atmospheric SO2 even at concentrations as low as 25 nl 1−1, with significant reductions in growth, reduction of C2H2, NH3 assimilation, protein synthesis, and heterocyst development. These disturbances appear to be mainly responsible for the extreme sensitivity of this fern to atmospheric SO2. Changes in violaxanthin/antheraxanthin and epoxy-lutein/lutein ratios also indicate that free radical products are induced by atmospheric SO2. These results suggest that the Azolla-Anabaena symbiotic system is a very responsive and reliable lower plant model to study the detailed effects of total sulphur deposition upon the balances between various important plant metabolic processes.