The aquatic fern Azolla is used as a green manure for rice production systems in warm temperate and tropical regions throughout the world. We used lead nitrate in nutrient media to assess the tolerance/response of two Azolla microphylla and Azolla filiculoides to heavy metals. While both species showed negative responses in growth with an increase in lead concentrations, They were distinctly different in their tolerance for higher concentration of the heavy metal. This was apparent in their growth and biochemical characteristics. A. microphylla was more tolerant of the higher concentrations of lead nitrate (25 and 50 ppm), whereas A. filiculoides had a higher rate of lead uptake.