During the austral summer of 1987/1988, three 24 h in situ primary productivity measurements were made at a nearshore sublittoral site on the east coast of Signy Island, Antarctica. The first experiment in December, coincided with the peak of the benthic algal bloom as shown by benthic chlorophyll measurements and a primary productivity rate of 700.9 mg carbon m−2 day−1. In January, the experiment was undertaken during the peak of the phytoplankton bloom when light intensities reaching the benthos were greatly reduced. A rate of 313.4 mg carbon m−2 day−1 was measured, half that of the previous month. In March the phytoplankton bloom had died off, benthic light intensities had increased and production was 391.8 mg m−2 day−1. The experiments indicate changes in benthic microalgal activity during the summer, linked to changes in the benthic light climate. Compared with previous measurements of phytoplanktonic activity at Signy, the microphytobenthos seems to be an important source of primary production. A production estimate of 100.9 mg carbon m−2, for the ice-free summer period, lies within the range of values of results from other polar studies.