The growth of juvenile populations of Ulva rigida C. Agardh was measured by means of immersion in in situ cages against environmental parameters (temperature, incident light, salinity, dissolved inorganic nitrogen and dissolved reactive phosphorus) in four different eutrophicated southern sites: Channel of the Thau lagoon (France), Lido, Sacca Sessola and Fusina stations (Venice lagoon, Italy). The growth curves as a function of temperature showed that, in all cases, the maximal temperature for Ulva growth was 17 degrees C (limitation in growth below 7 degrees C and above around 25 degrees C). The growth analysis of these four sites showed seasonal differences. In the least eutrophicated and calmest Lido station, grazing and dissolved reactive phosphorus (seven times lower at Lido than at Thau) played a key role. At Thau and Fusina, which are eutrophicated and turbid environments, the incident light had a strong impact on growth. Sacca Sessola, with an intermediate position between the above two mentioned situations, showed the highest growth rate. The values and relative growth rate (RGR) curves of the Mediterranean and open-sea northern sites are discussed. In particular, the temperature defines the type of growth curve (unimodal or bimodal) and the incident light is responsible for the low Mediterranean RGR values (<10% day(-1)).