On the north coast of Spain, Sargassum muticum is fertile between spring and summer. Maturation occurs gradually from April to September, Senescence occurs simultaneously with full maturity increasing from June to September. Differences between two consecutive years can be explained by changes in environmental factors, such as seawater temperature and nutrient concentrations. Assuming that receptacles are the only reproductive structures, standing reproductive allocation (standing RA) in the population varied between 20-24% and the maximum value recorded for a single plant was 55%. There was a positive relationship between standing RA and the size of the plant, but this allometric relationship was much clearer when the number of receptacles (NR) was plotted against the weight of the plant. This suggests that NR is fixed for a given plant size but that actual allocation (standing RA) varied among plants. Inter-annual differences between reproductive investment were related to environmental factors and other factors intrinsic to the population.