The East Carpathians Biosphere Reserve is located at the junction of Poland, Slovakia, and Ukraine; and therefore, lacks harmonized and detailed ecological regionalization encompassing all national parts. Thus, within the biosphere reserve and the vicinity, we delineated and classified morphogenic micro- and mesoecoregions as regional ecosystems, which spatially coincide with morphostructures of the 3rd and 2nd ranks respectively. The microecoregions were manually delineated using altitude and slope geodata derived from the Shuttle Radar Topography Mission Digital Elevation Model as a primary input, and national geological, geomorphological, and geoecological regionalizations as collateral inputs. Then, microecoregions were grouped into mesoecoregions and attributed with zonal statistics on mean altitude and mean relative elevation within a 1000-m circular neighborhood – both metrics were derived from the digital elevation model. Finally, microecoregions were divided into orographic classes according to the mean altitude and mean relative elevation metrics using agglomerative cluster analysis. We delineated 21 microecoregions and grouped them into five mesoecoregions. Overall accuracy of the resulting geodataset corresponds to a map of a 1:100,000 scale. Each microecoregion belongs to a certain structural-lithological zone (nappe), although the boundaries were modified by exogeneous processes. These are all flysch mountains, except one volcanic massif. Grouping of microecoregions into larger units – mesoecoregions – did not always follow hierarchy of geotectonic units. Cluster analysis on elevation metrics allowed to distinguish five orographic classes of microecoregions: 1) very low mountains, 2) low mountains, 3) dissected low mountains, 4) elevated low mountains, and 5) medium mountains. The regionalization and the classification reveal that mesoecoregions contain rather different microecoregions in terms of geological structure and orography. The latter also demonstrate rather significant internal heterogeneity. This study will be followed by a bioclimatic characterization and classification of microecoregions and subsequent descriptions of the potential natural and actual landcovers. Key words: East Carpathians Biosphere Reserve; ecoregions; morphostructures; orographic classes.