An important programme of biomonitoring along the coast of Algeciras Bay (Straits of Gibraltar) has allowed us tentatively to consider six species of boring sponges of the genus Cliona as biological indicators: Cliona celata, C. viridis, C. rhodensis, C. vermifera, C. schmidti and C. vastifica. The taxonomy and distribution of these species have been considered in this paper. Ecological obsevations have permitted us to identify some important characteristics of their habitat on the basis of local environmental parameters. Clioan viridis and C. celata withstand sedimentation higher than 7 gr/m2 and a solid suspension of 38 mg/l. Cliona vermifera and C. schmidti did not tolerate sedimentation higher than 1 gr/m2. One of these, C. vermifera Hancock, is recorded for the first time in the Iberian Peninsula.