A study of the population dynamics of Appendicularians in the Bay of Cullera has been carried out. Seven species were detected: Oikopleura dioica, O. longicauda, O. fusiformis, Fritillaria pellucida f. typica, F. formica f. digitata, F. borealis f. intermedia and Appendicularia sicu1a.The low level of diversity was associated with a high densities which occasionally reached 10000 ind m^-3. The effect of continental runoff, evaluated in terms of nutrient levels, is reflected in the distribution of the Appendicularian populations.