The mass distribution of total suspended matter was determined at 45 stations in the Gulf of Mexico using 0·45 μm membrane filters. In surface water, total suspended matter ranged from 600 μg/l. in the open Gulf to 2·4 mg/l. in near-shore waters. In water deeper than 100 m the range was 20 to 640 μg/l. A bimodal distribution was found which can not be completely explained; however, it appears that seasonal variations in primary productivity are important. The average value of total suspended matter for deep water was 166 μg/l., almost three times higher that the previously reported average.