AbstractIn the marshy fen country of N.W. Overijsel the hydroseres have been studied ecologically. Several successional sequences could be discerned, which develop as more or less independent and parallel series, but are cross‐connected by a continuous range of intermediate and transitional types of vegetation.In the diagram of these hydroseres the vertical direction from top to bottom represents the progressive succession, and the direction from left to right the increase in the degree of trophism and of various forms of disturbance and interference such as dunging and pollution.The ecological conditions prevailing in the successive stages of each hydrosere are described, much importance being attributed to the local fluctuations of certain environmental factors. This was substantiated by repeatedly taking water samples in the same spot during the course of the growing season. In many cases the maximum and minimum values of the range are of much greater significance than the average value. In this connection several species of bryophytes are valuable indicators.In the seres within the area under investigation, and more manifestly so in seres in other parts of Europe (including those occurring in more alkaline situations), several vicarious species (e.g., species ofDrepanocladus, Campylium, Riccardia, Pellia, Calliergon, Mnium, Bryum, Sphagnum, Eriophorum, Carex, Dactylorchis and Salix) can be recognised in the comparable phases of the hydrosere.Special attention is focussed on phenomena associated with seepage and their striking effects on the vegetatio