Eight Cocconeis Ehrenberg taxa (species and varieties) epiphytic on the leaves of Posidonia oceanica (L.) Delile were studied by scanning and transmission electron microscopy. Samples of Posidonia oceanica were collected throughout the year from seagrass meadows around the island of Ischia (Gulf of Naples, Italy). On the basis of some peculiar ultrastructural characteristics of the valve areolation, a new species, Cocconeis multiperforata sp. nov., and two new varieties, C. neothumensis var. marina var. nov., and C. scutellum var. posidoniae var. nov., are described. Ultrastructural analysis also revealed new morphological characteristics in C. britannica Naegeli in Kützing and C. maxima (Grunow) Peragallo & Peragallo, and confirmed observations of other authors for C. molesta var. crucifera Grunow, C. pellucida Grunow in Rabenhorst and C. stauroneiformis (Rabenhorst) Okuno. Our observations on the abundance of the different species provide new information on their distribution and ecology.