The dinoflagellate Karlodinium veneficum is a harmful algal bloom species with a worldwide distribution. This small athecate dinoflagellate makes a family of polyketide toxins that are hemolytic, cytotoxic and ichthyotoxic. The first chemical structure for karlotoxins from East China Sea (ECS) is reported here. The two new karlotoxins, namely 4,5-dihydro-KmTx 2 (compound 1) and 4,5-dihydro-dechloro-KmTx 2 (compound 2), were isolated and purified from monoalgal cultures of K. veneficum strain GM2. Their structures were determined by spectroscopic analysis, including tandem mass spectrometry as well as 1D and 2D NMR experiments. These new karlotoxin congeners feature a saturated polyol arm different from previously reported for KmTx 2 that appears to increase hemolytic activity.