Heavy metals are among the toxic substances for plants. Their presence causes a state called “heavy metals stress” which manifests itself by many physiological changes in plants. One of them is the formation of reactive oxygen species (ROS) which are scavenged by enzymatic and non enzymatic defending systems. The physiological responses of aquatic fern Azolla caroliniana Willd. at different mercury concentrations was investigated. During 9 days of the experiment under laboratory conditions, Azolla caroliniana was cultivated on medium enriched in 1, 20 and 30 mg Hg(II) dm-3. In this time the oxygen diffusion rate (ODR) and the superoxide dismutase activity (SOD) were determined. A strong release of oxygen by the fern roots (ODR up to 50 ľg m-2s-1) was recorded. It was related with an increase in SOD activities (up to 4000 units) above control values, which testifies to the existence of a defense reaction of A. caroliniana to the heavy metal stress.