The survival of Pseudomonas putida marked with the xylE gene was monitored in lake-water microcosms. Various designs of microcosms were compared. These ranged from 250-ml conical flasks containing 100 ml surface lake water to 12-1 glass containers with lake water overlying sediment, continuous aeration and a supply of fresh surface lake water. The presence of a low flow-through rate was shown to have little effect on the survival of P. putida. An increase in the size of microcosm, presence of sediment and aeration had a significant effect on survival in lake water and increased the rate of decline of released cells. The implication of these results in predicting the survival of P. putida in lake water using microcosms is discussed.