The epikarstic waters of a restricted sector of the Ojo Guareña cave (north Iberian Peninsula) were investigated to characterizethe physico-chemical variation in an annual cycle, to improve the scarce knowledge of the aquatic cave fauna on the IberianPeninsula, to look for distribution patterns of species per habitat along the annual cycle, and to search for the environmentalbasis (either physico- or hydro-geochemical) that could explain species distribution in the epikarst. The habitats studied includedall puddles and gours present in the sector selected making a total of 51 chemical and 42 biological samples for the entirecycle. The waters show no appreciable contaminationand exhibit small chemical variations throughout the year which arepatently affected by external weather conditions. The 53 taxa found belong to ten higher taxonomic groups (Oligochaeta,Turbellaria, Mollusca, Copepoda, Ostracoda, Isopoda, Bathynellacea, Tardigrada, Acarina and Cnidaria), and consist mainly ofcrustacea with a total of 27 species. Fourteen species were stygobionts (belonging to ten genera), nine of which are new to scienceand ten of which are endemic. Cave pools that appeared to be more stable in terms of water volume and mineralization, hada lower pCO2 and were carbonate oversaturated, harboured the greatest number of taxa. It is these pools that can maintain strictlycave dwelling species. Pools with lower levels of mineralization and greater water volume fluctuations had a lower diversityof fauna and in general lacked stygobiotic species.