Undaria pinnatifida, known as wakame in Japan, is cultivated in Asia in a multi-million dollar industry. It is primarily farmed for human consumption, with hundreds of different products made from it. In the 1980s U. pinnatifida was accidently introduced to New Zealand. It has subsequently spread on the east Coast of New Zealand from Stewart Island to the Hauraki Gulf. MAF Biosecurity classed Undaria pinnatifida as an “unwanted species” and so harvest or cultivation in New Zealand was illegal. In 2010 the policy changed and now it can be harvested and cultivated in some areas of New Zealand. It is prolific on existing mussel farms, prompting owners of mussel farms to apply for harvesting permits. Before this seaweed is widely commercialised in New Zealand, it is important to know about its basic biology and reproductive phenology. This study will address these issues for Undaria pinnatifida growing on mussel farms in the Marlborough Sounds.