This study investigates the influence of a temperature gradient on the maturation of fertile receptacles of Fucus vesiculosus. Samples collected from the North Sea at Sylt were cultivated at six different Temperatures (0°C – 25°C) over a period of five weeks. All other abiotic factors like light intensity, salinity and nutrient supply were kept constant. The number of conceptacles producing the gametes (eggs and sperms) mainly characterize the reproduction of this alga. The conceptacles are developed on the surface of the receptacles therefore their number and size are important parameters of the reproduction process. To investigate this in detail the following relationships were studied: • Receptacle growth and development with respect to the different temperatures • Number of conceptacles with respect to the different temperatures • Correlations between number of conceptacles and receptacle size It was shown that temperature has a significant influence on the maturation of the receptacles. The optimal reproduction temperature is in the range of 5°C – 15°C, where receptacles become biggest and develop the most conceptacles. Correlations between the number of conceptacles and the receptacle size were found. The number of conceptacles increases during maturation and is related to the growth of the receptacles. The description of this dependency with a linear model was not very accurate. But since it was shown that an increase in length, width, thickness and area of the receptacles is related to an increase of the number of conceptacles, these parameters could be used as derived parameters to determine the maturation level in the temperature range of 5°C – 15°C also in field studies.