During spring and summer 2003, we measured a variety of chemical and biological parameters in five medium-sized, Mediterranean cage farms that exploit semi-offshore conditions, and controlled the supply of fodder. The objective was to assess whether modern cage farms proliferating at semi-offshore sites exert environmental impact levels equivalent to the levels described from more traditional cage farms located in shallow, sheltered sites. In the water column, we examined the concentration of dissolved inorganic nutrients and heterotrophic bacteria in both surface and near-bottom water. At the bottom, we examined the concentrations of benthic chlorophyll a, phaeophytin and organic matter in sediments, the granulometric structure of the sediment, and the taxonomic (at the family level) abundance of benthic macroinvertebrates. For most parameters, we found no substantial differences between farm and control sites. Rather, most variation was explained as a function of depth (surface versus bottom water) or season (spring versus summer conditions). Deviations of farm values from control values, when they occurred, were small and did not indicate any significant impact on either bacterioplankton or benthic chlorophyll. Only one of the five farms studied exerted a detectable impact on the benthic macroinvertebrate community immediately under the cages. These results suggest that medium-sized fish farms located on semiexposed western Mediterranean coasts have fewer environmental impacts than traditional fish farms located in shallow, sheltered sites. Impact characterization in these new farms may require refinement of the standard approach to deal with rapid dispersal of effluents and sub-lethal levels of environmental disturbance.