Benthic blue-green algal mats in Lake Bonney, South Victoria-land, Antarctica, were investigated during the 1972-73, 1973-74 and 1974-75 austral summers. This investigation included (1) the distribution, structure, biological and chemical composition, and (2) measurements of photosynthesis, respiration and accumulation rates and modes of mats from the lake and associated meltstreams. The contribution of mat primary productivity to the total lake metabolism was estimated. Thin lemon-yellow mats of high algal diversity dominated by diatoms and filamentous blue-green algae comprised the stream mat community. These mats inoculated shallow areas in the lake undergoing glacial sediment influx where thick (up to 2.5 cm) laminated mats resided. The thick mats had orange upper surfaces and dark blue-green, sometimes anaerobic bottoms. Further from stream sources, mats were thinner, less laminated and more fractured. Mat pieces detached and rose into the water column buoyed by gases where they photo-oxidized, decomposed, or became frozen in new winter ice. Oscillatoriaceous algae (<i>Schizothrix, Oscillatoria</i> and <i>Phormidium</i> spp) dominated the thick mats. Diatoms (<i>Navicula</i> and <i>Nitzschia spp</i>) and coccoid green algae resided near mat surfaces and <i>Nostoc</i> sp grew throughout the mats. Thick mats photosynthesized at a range of 118-667 mg O₂/m²/12 hr and respired at a range of 100-460 mg O₂/m²/12 hr. Thin lake mats 2 photosynthesized at an average rate of 47 mg O₂/m²/12 hr and respired at a rate of 34 mg O₂/m²/12 hr. Thick lake mat fixed carbon at a range of 330-470 mg C/m²/12 hr with high dark bottle values, while stream mat fixed carbon at a range of 320-340 mg C/m²/12 hr with low dark bottle values. Comparisons of standing crop, carbon fixation rates and annual lamination patterns enabled identification of a variable growth pattern spanning 5-10 yr. Comparisons between lake mat and phytoplankton productivity data led to an estimated mat productivity contribution of 20-30% of total lake primary productivity. This investigation is the first to encompass all these aspects of benthic algal mat development in lakes and represents the only study yet conducted for Antarctica. / M.S.