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Cultures of four benthic diatom species (Navicula salinarum, Amphiprora cf, paludosa, N. arenaria, Gyrosigma spenceni) were kept at the surface of a mudflat in order to measure division rates. During incubations in the period of December to June the mean temperature was the predominant factor regulating cell division rate. Immersion of the mudflat with very turbid tidal water was an important factor only at a low-level mudflat station during winter and early spring. Strong temperature fluctuations during incubations, with periods of frost in winter and periods of insolation during hot summer weather, did not interrupt growth of the cultures. Hence high temperatures and high irradiance values are unlikely to be the direct cause of the summer minimum in population density that is frequently observed in the estuary. After the winter minimum the increase in population density was measured at three levels of the mudflat. At the high-level station the density increased in January at still very low temperatures, whereas at the mid-and low-level stations this increase started in March. This observation indicates that the emersion time of the mudflat and hence its exposure to full daylight is an important factor in the development of the spring bloom. The density of the diatom populations in the 0.5 cm top layer of the sediment increased only at a very low rate (with maximum rates of ca. 0.1 doublings d-l). Low photosynthetic rates of natural diatom populations in the sediment core are the cause of these low doubling rates of natural populations on the mudflat. Dispersal of diatom cells into the sediment is probably responsible for the decreased photosynthetic rates and, consequently, for the low doubling rates of sediment-inhabiting diatoms.