Ter Vell is a small (0.8 Ha) and shallow (usually less than 1 m deep) lagoon located in one of the ancient Ter River mouths. A one year (biweekly samples) study was carried out in 1984 to describe some limnological characteristics of the lagoon. Conductivity varied between 1 and 39 mS/cm. It was 1-3 mS/cm higher in the South basin (closer to the sea) than in the North one. Maximum values were observed in autumn-winter and minimum ones in summer. Phosphorus concentrations were most of the year over 4 u.g-at P-PO;-/I. The highest concentrations (50 ^Lg-at/I) were observed in summer. Phosphorus concentrations from October to May were positively correlated to conductivity, while those from May to September were not. Oxydized dissolved inorganic nitrogen was very low all the year round (less than 4 pg-at/I). Phytoplankton assemblages were between 104 and 4.105 cells per ml mostly because Nannochloris sp., which was overwhelming during all the year. Accompanying species were Monoraphidium sp. pl., Actinastrum hantzschii, Dictyiosphaerium pulchellurn, Cyclotella kutzingiana, Lepocinclis ovum and Ochromonas sp. pl. during summer, and Dunaliella sp., Hemiselmis rufescens, Pseudopedinella pyriformis, Katodinium rotundatum and Oxyrrhis marina during winter. The amount of chlorophyll a per cell was always very low (between 0,1 and 2 pg per cell). During summer the most abundant zooplankters were Brachionus sp. pl., Moina micrura, and Acanthocyclops g. vernalis. During the cold months Notholca squamula, Synchaeta sp., and Epiphanes brachionus were the most abundant. Maximum abundances changed from 2,000 individuals/I in summer to 600 individuals/I in winter. Ter Vell is an eutrophic lagoon with marked differences between chemical and biological characteristics of different periods of the year. Two main periods can be distinguished related to temperature. External factors, mainly freshwater and sea water inflows, also control the differences observed