Dentrifying mesophilic and thermophilic bacteria were looked for in rice paddies of Senegal by culturing samples in medium with high concentrations of nitrate or nitrite (5 g/l) as respiratory substrate. These cultures revealed the existence of two populations. 1) a population of denitrifying mesophiles which tolerate high concentrations of nitrite, these organisms being mostly spore-formers and relatively numerous in these soils; growth studies showed them to be very diverse: a) "nitrite-dependent" bacteria unable to reduce NO-3; b) bacteria highly nitrite-tolerant with rapid growth on 5 g/l nitrite; c) bacteria slightly nitrite-tolerant with weak or no growth on more than 3 g/l nitrite; d) NO-tolerant bacteria which use nitric oxide as respiratory substrate for growth; e) N2O-deficient bacteria unable to grow on nitrous oxide; 2) a population of denitrifying thermophilic spore-formers which are numerous in some soils and tolerate nitrite more or less well. Measuring of the denitrifying activity of washed cells demonstrated that, in general, cells grown anaerobically on nitrite show much more activity than cells grown anaerobically on nitrate. This nitrite-tolerant population seems fairly heterogeneous, but it consists mostly of spore-forming species of the genus Bacillus.