Some physical data collected over a period of a year in seven locations of the Ogof Ffynnon Ddu cave system in South Wales are reported, including humidity, air and water temperature, pH of the water, as well as the organic oxygen demand of the water. It is shown that seasonal variations in the physical constant in this particular cave system are not well marked. Algae and bacteria were isolated from the soil samples and from calcareous deposits. A total of 30 algal species, of which 13 belong to the Cyanophyta, 22 to the Chlorophyta, and 7 to the Chrysophyta-Baccilariophyceae were found. Thirty-eight heterotrophic and 7 autotrophic bacteria were isolated. The thin films on water surfaces, besides diatoms, contained several flagellates and some ostracods, while some protozoa were found associated with the bacteria and algae in the soft calcite deposits.