As a result of the groundwater level decline observed in the last two decades and the socio-economic changes, we assume that a drastic alteration begins in the landscape pattern of Danube-Tisza Interfluve (Duna-Tisza köze, Kiskunság s. l.). It demands the documentation of the actual state of vegetation in this region, and its quantitative analysis, as well. In the first of the two articles presenting our results we discuss the regional habitat-pattern of the landscape, the background factors determining it, and the basic features of fen and alkali vegetation. In the second part, this will be completed by the sand, riverine and steppe vegetation and the data on habitat-devastation on the regional scale.During our study we determined the Danube-Tisza Interfluve (with a total extension of nearly 1.4 million ha) in the geographical sense. The analysis of the present habitat-pattern of the region was based on data of the actual (1996–2000) habitat map of Danube-Tisza Interfluve (Molnár
2000) and on our experience during the field survey. We compared our data to surface-geographical, pedological and hydrological works, to maps from the 18–19th centuries on land-use, to botanical studies accomplished in the 19–20th centuries and to the results of other related sciences. The quantitative analysis of the vegetation pattern was carried out considering the vegetation subregions.About half (49.4%) of the surveyed, not cultivated habitats of the Danube-Tisza Interfluve is in a natural, semi-natural state, and nearly a quarter of them (23.8%) is essentially disturbed or under deep human impact. During the one- and-a-half decades between the topographical mapping of the mid-1980s and the habitat mapping (D-TMap), 14.7% of the surveyed habitats disappeared, and appr. 12.1% is now in the state of regeneration after the disturbance of the distant or the recent past. Today, fen vegetation (including the vegetation of non-alkali swamps and uncharacteristic fen vegetation that is drying out at present) is predominant among the studied habitats of the region (it covers 95,135 ha), that is followed by alkali vegetation with an extension of about half of the former one (47,226 ha).The joint vegetation categories (sand, fen, alkali and riverine vegetation) of the region compose vegetation zones running north and south. So, on the two sides of the Sand Ridge vegetation zones dominated by fen habitats lie, which is followed by the zone of alkali habitats; finally, the riverine vegetation prevails along the rivers Danube and Tisza. From the abiotic factors determining these zones we emphasised the hydrodynamic characteristics — that play recently the most essential role. As a result of the analysis of landscape pattern, it became obvious, that the natural habitats — similarly to the hydraulically continuous groundwater flow systems — of the Danube-Tisza Interfluve compose hierarchical systems, basically on three different levels (on the local, intermediate and regional scale). In this point of view, those areas are considered as a certain system, where the underground water flow (recharge, throughflow and discharge) forms a uniform flow system. We approached to the survey of the actual vegetation of the Danube-Tisza Interfluve with this viewpoint of the hierarchically structured habitat pattern.