ABSTRACT: A pulse amplitude modulated fluorometer (PAM) was used to investigate photosynthetic activity of microphytobenthos on an intertidal mudflat. Spectral irradiance measurements indicate that 75% of the signal detectable by the PAM originates in the upper 150µm of the sediment. From the photosynthetic electron transport rate (ETR) measurements, it was concluded that the PAM could be used to observe changes in photosynthetic parameters during the day or the season. Photoacclimation to lower irradiancewas indicated by changes in the maximum ETR and the saturating photon irradiance parameter I_k. When cores were exposed to a high photon irradiance for several hours, vertical migration could be followed using reflectance spectra.The data also showed that the benthic algae did not seem to experience photoinhibition or CO₂ limitation. To explain this, it is hypothesised that there is a continuous vertical migration in the top layer of the sediment, where algae can avoidphotoinhibition due to prolonged periods of high irradiance and lack of CO₂ by migrating downwards while others migrate upwards.