To establish the growth rate of Pistia stratiotes L., two sets of experiments were conducted. In one set, two wire baskets, approximately l m 3 in size, were kept immersed, one at Zone A and the other at the farther end of the lake where water is taken out for treatment and supply to the community. The baskets helped in preventing free movement of the plants. Ten plants were kept in each basket. The plants were weighed at intervals and the results indicate that the growth rate was very high at Zone A, where the pollution was at a maximum. In 17 days, ten plants grew into 18, with an increase in fresh weight of 78 g. In the other zone, the number remained the same. In another set of experiments two 1 m 3 pits were dug and filled with lake water. One of the pits was provided with decaying plants of Pistia. Five Pistia plants were introduced into each of the pits and their growth rate was observed over a seven-week period. At the end of the first week the numbers increased to 44 in the pit which received decomposing Pistia plants. At this point all but five plants were removed. In the next three weeks the numbers increased to 48. At this point all but five of the plants were again removed. In the next three weeks the numbers again increased to 52. In the control pit the numbers remained constant throughout. On average, the number of plants multiplied four times in seven days.