New Zealand experienced its first recognised outbreak of shellfish poisoning around Northland in early 1993. The poisoning symdrome was characterised as neurotoxic shellfish poisoning (NSP) from the observed symptoms. The major causative toxins were isolated and characterised by the Japanese research groups of Professor Ishida (University of Shizuoka) and Professor Yasumoto (Tohoku University). The toxins were identified as brevetoxin B derivatives. Diarrhetic shellfish poisoning (DSP) was suspected as responsible for further poisoning around this area. Yessotoxin (YTX), a suspect DSP toxin, was isolated as part of this research and is the first reported instance of YTX in New Zealand. A stereochemical study was carried out on a desulfated YTX derivative. The results suggested anomalies in the reported conformation of YTX. Attempted synthesis of haptens of YTX for development of an ELISA is outlined in this thesis. Another incident of shellfish contamination occurred around Foveaux Strait in early 1994. The toxicity was characterised by NSP symptoms in mice. The causative toxin, gymnodimine, was first reported by a Japanese research group, although independent parallel structure elucidation was carried out as part of this research. The absolute stereochemistry of gymnodimine was solved by X-ray analysis of a synthetically modified derivative. Gymnodimine haptens were prepared for development of an ELISA. A chromatography based assay has been developed for the specific detection of gymnodimine in shellfish and alga. This work was modified from the extremely efficient isolation procedure for gymnodimine developed as part of this research.