Two dives, to depths of 7,300 m and 3,100 m were performed on the southern side of the Puerto-Rico trench, chiefly for biological observations. The deposition of organic material is more considerable at 7,300 m (where a process of decantation seems to occur) than at 3,100 m. Limestone flags observed at 3,100 m showed signs of active present-day corrosion.Plankton study is made difficult, when the bathyscaphe dives to great depths, by the speed of descent (about 1 m/sec). During the dive to 7,300 m the density of the plankton showed a marked diminution towards 6,600 m, that is, passing from the abyssopelagic to the hadopelagic zone. The increase in density of the plankton usually observed in the last few metres above the bottom seemed slight, although many amphipods (Stegocephalids ?) were seen. During the dive to 3,100 m numbers of a medusa (Aglantha ?) were seen between 1,500 and 2,800 m; below 2,900 m plankton was virtually absent.Only the benthic epifauna could be studied, as the bottom sampler failed to work. At 7,300 m the community was characterized by the abundance of a liparidid fish (Careproctus ?: about 200 individuals); fair numbers of holothurians were also seen (Synallactidae and Myriotrochus), several Nematocarcinus, and a large asellote isopod, probably of an undescribed species. At 3,100 m a total absence of epifauna was noted on hard substratum, and a synallactid holothurian was dominant on muddy sand; the fishes were very rare.General remarks on the use of the bathyscaphe, in its present state, for biological work, are given.