In vitro short time experiments (a few weeks) show that daily ingestion by Paracentrotus lividus (Echinodermata: Echinoidea) is weak (0-81 mg DW d(-1) ind(-1)) for Caulerpa taxifolia (Chlorophyta), compared to the data recorded with an almost totally avoided brown alga, Cystoseira compressa (24-137 mg DW d(-1) ind(-1)) and with a preferred brown alga, Halopteris scoparin (79-397 mg DW d(-1) ind(-1)). There is no significant difference between spring (when the toxic metabolite content is at its minimum) and summer-autumn (when it is at its maximum). In contrast, the absorption rate of the sea urchin fed on Caulerpa taxifolia is significantly weaker in spring than in summer-autumn. Overall, the daily absorbed weight is negative in spring with C. taxifolia (compared with 18 mg DW d(-1) ind(-1) with Cystoseira compressa and 25 mg DW d(-1) ind(-1) with H. scoparia). It has also been demonstrated that P. lividus becomes accustomed to Caulerpa taxifolia, at least for short time experiments: the echinoid consumes greater quantities of this alga especially if it is fresh when it has already consumed it previously.