For pest management or introduced marine species to succeed, a thorough understanding of reproductive patterns is essential. Undaria pinnatifida is an invasive macroalga that has been introduced into at least 10 countries. Reproductive phenological studies in Tasmania, Australia, were undertaken to provide much-needed quantitative information to support pest management. Zoospore release or U. pinnatifida, an annual kelp, was limited to the larger size classes of sporophytes (> 55 cm length) for most of the growing season, with the proportion of mature sporophytes increasing towards the end of the season. Small sporophytes with mature sporophylls were not observed until late in the growing season, i.e. after November. The maximum zoospore release of U. pinnatifida was 62 X 10³ zoospores cm² sporophyll tissue h¹, corresponding to a maximum release of 4.3 X 10 8 zoospores sporophyte ¹ h¹. Spore release rates of other kelp species are similar or higher, the latter especially in larger-sized species. Tagged U. pinnatifida individuals in the present study released zoospores for about three months before becoming senescent and disintegrating. Hypothetically, the smallest mature sporophyte would have a stipe width or 0.6 cm, corresponding to about 33 cm in total length, with a sporophyll circumference of 7.6 cm and a sporophyll biomass of 0.2 g. The zoospore release of an assemblage of introduced U. pinnatifida at the study site was estimated as 2 X 10 9 zoospores m ² h ¹ in the month of January (summer). The two largest size classes released the majority of zoospores. Management efforts involving the manual removal of U. pinnatifida to control this species could be rationalized by concentrating on the removal of only larger sporophytes (> 55 cm), potentially resulting in significant cost savings.