INTRODUCTION During the summer months of 1962-1964 an extensive survey, covering several biological, oceanographical and sedimentological subjects, was made in the Ria de Arosa (Galicia, N.W. Spain) by a scientific team from the University of Leiden. Numerous observations and intensive sampling yielded valuable data for detailed studies of, for instance, Foraminifera, Diatomacea, Mollusca, sedimentology, etc. For a detailed summary of the investigations undertaken the reader is referred to Brongersma & Pannekoek, 1966. The present report deals with the ostracod fauna recovered from a number of samples taken in brackish and marine environments of this bay. The material was placed at the author's disposal through the courtesy of Drs. M. Brongersma-Sanders (Rijksuniversiteit, Leiden) and J. H. van Voorthuysen (Rijks Geologische Dienst, Haarlem). All samples studied have been stored with the collections of the Rijksmuseum van Natuurlijke Historie, Leiden. The author wishes to express his sincere gratitude to Mr. D. ter Keurs (Bataafse Internationale Petroleum Maatschappij N.V., The Hague), who performed a preliminary identification of the ostracod species. Mr. L. R. Funcken (Heerlen) prepared the photographs of specimens coated with ammonium chloride. TAXONOMIC PROBLEMS According to Ter Keurs at least 43 species are present in the Ria de Arosa. Only part of these have been identified with absolute certainty ; most of the species here are either compared with similar forms from elsewhere, or are presented in "open" nomenclature. As pointed out by Noordermeer & Wagner (1969) ostracod taxonomy has reached a chaotic state in the last decades.