A study was made of the taxonomy, distribution, and ecology of the Phaeophyta of the eastern Gulf of Mexico, a region defined as the coastline and adjacent continental shelf from the Mississippi Delta at Barataria Pass, Louisiana, to Dry Tortugas, Florida. This coast, more than 1000 miles long, has been the least known in the western North Atlantic Ocean with respect to algae. The principal environmental factors influencing the distribution and abundance of the algae in the area vary in such a way as to divide the region rather naturally into six subregions: Subregion A. Mississippi Delta to Pensacola. Characterized by dissected shoreline of marshes and smooth sand beaches; numerous rivers; muddy bottom; turbid water; wide range of salinity and temperature. Subregion B. Pensacola to Cape San Blas. Characterized by smooth shoreline of barrier islands and sand beaches; sandy bottom; clear water; stable, high salinity; wide temperature range. Subregion C. Apalachicola to Anclote Keys. Characterized by dissected shoreline of marshes and rock islands; rocky bottom; water turbid to clear; numerous rivers; wide salinity and temperature range. Subregion D. Tampa Bay to Cape Romano. Characterized by shoreline of barrier islands; sandy bottom; rocky outcroppings; “beach rock”; clear water; high salinity; moderate range of temperature; large estuaries with variable turbidity, salinity, and temperature. Subregion E. Cape Romano to Florida Bay. Characterized by dissected shoreline of mangrove swamps and islands; calcareous mud-sand bottom; clear or milky-green water; high, stable salinity; narrow temperature range. Subregion F. Florida Keys. Characterized by numerous islands; coral reefs and limestone; clear or milky-green water; high, stable salinity; narrow temperature range. Data from many sources are compiled to show conditions of temperature, salinity, and light in the eastern Gulf of Mexico, and the diversified substrata are described, with lists of species given for each habitat. Two new terms are introduced to characterize species with regard to their range of potential substrata: euryancyric, denoting species which attach to diverse substrata, and stenancyric, which applies to species with a narrow range of possible attachment sites. During summer, inshore temperatures from the Mississippi Delta to the Florida Keys average 28–30°C., and the Phaeophyta present are tropical, with quantity and diversity gradually diminishing in a northward direction from the Florida Keys. In winter, inshore water temperatures from Cedar Keys to the Mississippi Delta average 13–15°C. Tropical species disappear or are reduced in the northern Gulf during this season, but at least 13 species of Phaeophyta characteristic of cold water are temporarily abundant. Most of these species are present also along the northeastern United States, but are not continuous around the tip of Florida. Species with disjunct distribution probably are remnants of a flora that was continuous along the Atlantic and Gulf coasts across submerged northern Florida during the Pleistocene epoch. Although there is no sharply defined temperature break marking northern and southern distribution patterns in the eastern Gulf, the Tampa Bay area (average winter temperature 18°C.) is the northern known limit for eight tropical species of Phaeophyta and the southern known limit for seven temperate species. Seventy-two species of Phaeophyta are recorded from the region, including two new species, Sargassum wolfii and Padina profunda. Numerous range extensions are given, and 64 species are for the first time characterized with regard to temperature, salinity, and substratum range, based upon field measurements and inference from distribution. The proportion of Phaeophyta in the total algal flora can be used as an indication of floral affinities in the western Atlantic, with 12–15% characteristic of tropical regions, 20–25% characteristic of temperate regions, and 30% or more characteristic of distinctly cold-water regions. The algal flora of the Florida Keys contains 13.2% Phaeophyta and is tropical, whereas the northern Gulf flora, with 24.6% Phaeophyta, has temperate affinities.