Research of processes of secondary hydromorphism of soils presents considerable difficulty. Up to date the isolated diagnostic criteria of classificatory compliance of soils of secondary over-wetting widely spread in the Northern Caucasus and especially in the piedmont Western Ciscauca-sia haven't been set. In 2003-2004 research was conducted with the aim of identification of changes features of biological properties of different black humus soils of Northern Caucasus upon waterflooding. The registered changes can help in assessment of bioindicators of different degree of hydromorphism as the natural overwetting of soils can be manifested to a different de-gree depending upon climatic conditions and very often it doesn't leave traces in the morphology and chemical quality of soils. In the research, traditional methods of assessment of biological properties were used. The most common soils of the region under study were employed. The soils for model experiments were collected from the topsoil (0-30 cm) of common black carbon-bearing humus of the Krasnodar region (village of Plastunovskaya), Rostov region (“Donskoye” work-study unit) as well as leached black humus soil (city of Krasnodar). The basic diagnostic criterion for these soils is car-bonic content in the upper part of section. In glass jars in three replications the natural flooding with tap-water was modeled. The expo-sure term was 30 days. Biological properties of black humus soils differ considerably for check variants of carbon-bearing humus and leached black humus. For example, in the leached variant of black humus the check value of catalase activity that characterizes oxidizing conditions, is twice as little than in the common carbon-bearing variant. Upon waterflooding the differences in the behavior of catalase activity are aggravating. In both carbon-bearing soils, irrespective of the place of sampling, upon flooding the considerable proved inhibition of catalase activity is evidenced ( by 16-27%). At the same time upon flooding of the leached black humus its increase by 32% is registered. The activity of invertase upon flooding of the leached black humus is decreased slightly, and that confirms our statements about low application of this parameter in the diagnostics of hydromorphism processes. The waterflooding definitely subdues the development of microbial flora of black humus soils in all the variants. The most decrease was registered in the numerosity of microscopic fungi (reduction by 59-84%), the numerosity of bacteria was decreased to a lesser degree (by 19-69%). Sustainability of microorganisms of the leached black humus is significantly lower than that of the carbon-bearing humus. Upon flooding the biological activity indexes are also decreased. Pulp decomposition declines considerably (almost by half). This is a well-known fact with which a significant accumulation of uncut moquette in the form of peat in swamps is connected. Decomposition of urea also declines and that indicates the intensity reduction in nitrogen cycle processes. The result is that the possibility of usage of biological indexes in diagnostics of soil waterflooding processes was discovered. Based on the change of biological parameters, different sus-tainability to the given type of degradation of automorphous black humus soils was registered. Carbon-bearing humus soils are more sustainable to overwetting. Leached black humus soils are more susceptive to the given effect.