In the Tulenovo oil deposit, Northeastern Bulgaria, waters contaminated with crude oil and toxic heavy metals (Cd, Cu, Ph, Mn, Fe) were treated by a passive system of the type of the constructed wetlands. The waters had a pH of about 5.3-6.8 and contained about 2-10 mg/l oil. The concentrations of heavy metals exceeded by two to eight times the relevant permissible levels for waters intended for use in the agriculture and/or industry. The flow rate varied in the range of about 0.1-0.5 l/s. The wetland covered an area of about 200 m(2) and was about 35-40 cm deep. The wetland contained abundant water and emergent vegetation and a varied microflora. Typha latifolia, Typha angustifolia, Phragmites communis, Scirpus lacustris, Juncus spp. and different algae were the main plant species in the wetland. The microflora consisted of different hetero-trophic and autotrophic microorganisms. including different oil-degrading bacteria and fungi, The oil content of the waters after treatment was decreased to less than 0.2 mg/l, and the concentrations of heavy metals were decreased below the relevant permissible levels. The removal of the oil was connected with its degradation by the indigenous microflora. The removal of the toxic heavy metals was connected mainly with the processes of microbial dissimilatory sulphate reduction and biosorption,