Bioassay-guided fractionation of the organic extract of a Curaçao collection of Lyngbya majuscula led to the isolation of a new lipid, curacin A, with exceptional brine shrimp toxic and antiproliferative activities. Its unique thiazoline-containing structure has been deduced from spectroscopic information. Pure curacin A is an antimitotic agent (IC60 values in three cell lines ranging from 7 to 200 nM) that inhibits microtubule assembly and the binding of colchicine to tubulin.