Recent research suggests that marine organisms may produce compounds with activity against malaria parasites. Of a total of 27 aqueous extracts from different marine species, collected on the northwest Cuban coast, 20 were considered as showing no significant activity against Plasmodium falciparum F32, with minimum inhibitory concentrations (MIC) >500 microg/ml, while seven extracts (MIC < or =500 microg/ml) were selected for further investigation by determining their selectivity indices and in vivo antimalarial activity. Three species of tunicates were chosen, as more than 50% reduction of P. berghei parasitaemia was produced after administration of 250 or 500 mg/kg of their crude extracts into infected mice. The aqueous extracts of Microcosmus goanus, Ascidia sydneiensis and Phallusia nigra were partitioned between water and n-butanol; the organic phases inhibited P. falciparum growth by 50% at concentrations of 17.5 microg/ml, 20.9 microg/ml and 29.4 microg/ml respectively. In general, these results are similar to those of most ethnobotanical surveys. Further chemical studies are being undertaken in order to isolate new antimalarial compounds from these Caribbean tunicates.