Denitrification was studied in the sediment of a shallow, Danish estuary in the spring and summer of 1982 and 1983. The acetylene blockage technique was used in undisturbed cores to estimate the in-situ activity of the process. A declining rate of denitrification was observed at the onset of the growth season when the allochthonous input of nitrate levelled off; the denitrification decreased from 5.1 mmol N m -2 d -1 in the early spring (April) to 0.3 mmolNm -2 d -1 in the early summer (June). The activity in the uppermost centimeter comprised more than 70% of the total. In this surface zone, a diurnal pulse of oxygen production from photosynthesis of benthic microalgae was found to affect the denitrification; the latter had a distinct minimum in the day when the oxygen production was high and inhibitory. The subsequent, nocturnal stimulation of the denitrification was typically a factor of three or more in the investigated periods. We propose that oxygen is an important control factor for the daily, in-situ denitrification in shallow estuaries where light is adequate to reach the sediment. The availability of nitrate seems more important as a control factor for the seasonal variation of the process.