Globally, north-south gradients in diversity are well known, and in the world ocean theyhave been related to north-south gradients in water column structure. Here diversity in planktonicciliates, both taxonomic and morphological, is explored in the Mediterranean Sea, which displayswest-east gradients in the water column characteristics of chlorophyll concentrations and distributions.Tintinnids, which are loricate planktonic ciliates, were enumerated and identified in samplesfrom 23 stations between 2 and 32° E, obtained from the Catalan Sea in June 1993 and from a cross-Mediterranean transect in May-June 1996. From west to east, concentrations of tintinnids showed littlevariability, while numbers of species and genera, as well as diversity indices (H’ values), increased.These parameters were positively related to depths of the chlorophyll maximum layer and negativelycorrelated with chlorophyll concentrations. Corresponding to the west to east increases in diversitywere increases in community averages of lorica oral diameter and overall length, again positively correlatedto deepening of chlorophyll maximum depths, and negatively related to average chlorophyllconcentrations. Morphological diversity was related to species diversity, not as variability in lorica oraldiameters, but as variability in average lorica length. In the Mediterranean Sea, a gradient of taxonomicand morphological diversity is relatable to a specific feature of the water column, the depth ofthe chlorophyll maximum layer. Trophic specialization as a dominant mechanism underlying diversitywas not supported by data on lorica oral diameters.