Kelp aquaculture is a growing industry in Australia and to ensure optimal operations, nurseries that can supply high quality seedlings all year around are necessary. To optimise the nursery stage (shortened cultivation time or improved output) detailed knowledge about the physiology of the cultured microscopic life stages are essential. We investigated the impact of six zoospore densities (10-278 individuals per mm2) on the subsequent development of Ecklonia radiata gametophytes. Germination rates and sex ratio were unaffected by initial zoospore density. Female gametophytes after two weeks were largest at initial zoospore densities of 40 mm-2 with smaller individuals in higher and lower densities. This pattern did not persist and after four weeks female gametophyte size and cell number significantly increased with decreasing initial zoospore density. Significantly more sporophytes developed at initial zoospore densities below 40 individuals mm-2 and no sporophytes were observed at the highest density (271 zoospores mm-2). These results highlight the importance of investigating gametophyte size relationships at different stages in the development process and clearly show the importance of initial zoospore density in optimising the nursery stage of kelp aquaculture.