Antimicrobial, anti-diatom and anti-larval activities of both water soluble (water extracts) and non-water soluble metabolites (methanol: chlorophorm, 1:1 extracts) of the sea cucumbers Holothuria atra and Holothuria edulis from Bander AL-Khiran region, Oman were tested in this study. There was no significant effect of the extracts from sea cucumbers on bacterial (3 reference bacteria from seawater and pathogens Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus epidermidis) and the diatom Chaetoceros sp. growth. Both water extracts and methanol: chlorophorm extracts caused significant mortality of Artemia salina nauplia. This study suggests that Omani sea cucumbers might be a good source of toxic anti-larval compounds.