Yessotoxin has been responsible for several shellfish toxicity events within New Zealand. The dinoflagellate Protoceratium reticulatum is thought to be responsible for the occurrence of this yessotoxin. We aim to establish at what stages of a P. reticulatum bloom yessotoxin toxicity is the greatest and to find out which derivatives of yessotoxin are produced by P. reticulatum at different stages of growth and under different environmental conditions. This information is important for effective phytoplankton monitoring programs. We have established standard culture conditions and measured cell growth under varying environmental conditions for P. reticulatum. The most effective growth medium was GP, a seawater medium containing soil extract. The combination of light intensity and temperature that produced the most rapid growth was 45µmolm-2s-1 and 21OC. The lag, exponential and stationary phases of growth have been defined. Culture samples of P. reticulatum are now being analysed by HPLC to quantify the amounts of yessotoxin and its derivatives produced at different growth stages and in various environmental conditions. The results of this analysis will be presented.