A total of 24 Alvin dives were made at 10 sites. Chemosynthetic communities were found at all of the sites, along with hydrocarbon seepage, carbonates, and occasional brine seeps. Seep-related megafaunal communities were sampled at each site. Bathymodiolid mussels, representing at least four different species, were sampled from every site visited. Vestimentiferan tubeworms were visually confirmed at every site except one, and the tubeworms were collected from seven sites. Four of the 10 sites had diverse and densely populated seep communities and topographically complex sea-floor geology. These four sites became our key sampling locations: Atwater Valley Block 340 (2,200 m depth), Green Canyon Block 852 (1,410 m depth), Alaminos Canyon Block 601 (2,340 m depth), and Alaminos Canyon Block 818 (2,740 m depth). Two of the most important chemosynthetic faunal groups are the bathymodiolid mussels and vestimentiferan tubeworms. Prior to the Alvin cruise, the depth occurrence interval for Bathymodiolus childressi was established with a range from 500 to 2,200 m. For another mussel, Bathymodiolus heckerae, sample collections made during this cruise extended the depth range for this species. Previously, this species had never been observed shallower than 2,220 m and was never found with B. childressi. The mussel with the widest depth range was found to be Bathymodiolus brooksi, which is the most common mussel found in middle and lower slope depths. Based on tubeworm collections, Escarpia laminata was found to be the dominant species below water depths of about 1,400 m. An undescribed species of tubeworm, Lamellibranchia sp., was found to be abundant on the mid-slope between water depths of 1,100 to 2,300 m. Regarding slope-wide faunal changes, our collections indicated that along-slope changes were not significant, but depth-dependent changes were particularly apparent in the mussels and tubeworms. The communities of organisms associated with tubeworms and mussels at lower-slope seep sites were found to be quite different than upper slope (less than 1,000 m) communities. Gastropods and mollusks are dominant on the shallow slope while brittle stars, holothurians (sea cucumbers), and heart urchins are most abundant in middle-to-lower slope depths. Gorgonians were observed at most sites, while large colonies of bamboo corals were present at Alaminos Canyon 645. An extensive coral community, including Scleractinian and hard corals and several octocorals, were observed on large carbonate boulders at Green Canyon 852. A voucher specimen of the hard coral, Enallopsammia rostrata, was collected at a water depth of 1,397 m, which extended the depth range of this species. Another interesting find was a large brine lake at one of our deepest sites, Alaminos Canyon 601. The brine lake was approximately 4 m deep and 180 m wide. The salinity of the lake brine was about 2 times that of sea water, and white "flocs" were found floating in the brine. The white "flocs" were observed on the lake bottom and concentrated at the "shoreline." This white material was later determined to be barium sulfate (barite), a high specific gravity mineral used in drilling mud. However, this occurrence of barite is natural and did not result from drilling for oil and gas. There was no animal life visible in the brine, and after analyzing core samples, researchers found no animal life at the brine lake bottom, either. Heart urchin and mussel communities were distributed in patches around the lake edge. Results from this cruise confirm the widespread existence of hydrocarbon seep habitats and their chemosynthetic communities on the middle and lower parts of the continental slope. The cross-slope and along-slope variability in the geology and geochemistry of these deep-sea seeps suports a surprisingly high density of biological communities. Hydrocarbon seepage sustains life to the deepest parts of the Gulf of Mexico, and highly productive chemosynthetic communities are not limited to water depths shallower than 1,000 m.