ABSTRACT: Ostracod assemblages of 52 samples, collected between 15 and 50m water depth on the Southern Tuscany continental shelf (Northern Tyrrhenian Sea), were analysed. A total of 68 species and 39 genera was determined. Q-mode cluster analysis performed on the 22 most abundant species (>5% in at least one sample) led to identification of clusters (and subclusters) corresponding to four distinct ostracod assemblages. The first assemblage is characterised by the high dominance of Cytheridea neapolitana and is probably linked to an organic matter enrichment, in silty clay bottom; the other three assemblages are more diversified and are dominated, with lower percentages, by species typical of infralittoral and circalittoral environments: C. neapolitana, Cytheropteron ruggierii and Loxoconcha subrugosa assemblage (on silty clay sediments), Pontocythere turbida assemblage (silty sand and silty clay substrates) and Loxoconcha ovulata assemblage (sandy clay and vegetated bottoms). The distribution of ostracod assemblages shows a good correlation with the benthic foraminiferal assemblages founded in the investigated area. Nevertheless, the recognised low diversity can be related to an organic matter enrichment.