Coolia monotisMeunier (Dinophyceae) was isolated from drift seaweed collected from Ninety Mile Beach, Northland, New Zealand. It is a new record for New Zealand waters. The microalga was identified by scanning electron microscopy and cultured in a seawater‐based medium. Cultures grew preferentially at 25°C (subtropical temperatures), rather than at 20°C (temperate), and at salinities >28.Coolia monotisproved to be toxic to larvae ofArtemia salinaandHaliotis virginea.