As part of our continuing research on seaweeds, crude MeOH extracts of two green, three brown and six red algae collected from Marmara, Black, Aegean and Mediterranean Seas were screened. Four parasitic protozoa, i.e. Plasmodium falciparum, Trypanosoma brucei rhodesiense, T. cruzi, Leishmania donovani and the tubercle bacillus Mycobacterium tuberculosis were used as test organisms for the in vitro assays. The selective toxicity of the extracts was also determined against mammalian L6 cells. All seaweed extracts were active against T. brucei rhodesiense; the Dasya pedicellata extract was the most potent (IC50 value 0.37 mu g/mL). The same extract also weakly inhibited the growth of T. cruzi (IC50 62.02 mu g/mL). All seaweed extracts also showed leishmanicidal activity (IC50 values 16.76-69.98 mu g/mL). The majority of the extracts also exhibited antiplasmodial potential and the most potent extracts were those from D. pedicellata (IC50 0.38 mu g/mL), Codium bursa (IC50 1.38 mu g/mL) and Caulerpa rasemosa (IC50 3.12 mu g/mL). One brown and two red algal extracts showed some weak activity against Mycobacterium tuberculosis (MIC values 125-256 mu g/mL). Except for the extract of Dasya pedicellata, none of the extracts displayed any cytotoxicity. This is the second study investigating the antiprotozoal activities of Turkish marine algae and identifies Dasya pedicellata, an understudied algal species, as a candidate for further studies. Copyright (C) 2010 John Wiley & Sons, Ltd.