A population of the sipunculan worm Aspidosiphon muelleri, located in temperate waters of the western Mediterranean Sea, was monitored monthly for a year. Some aspects related to its ecology and reproductive biology are shown in the present work. The sex-ratio for this population was close to 1:1 (54% females vs 46% males), thus indicating a dioecious reproduction, although showing a lack of sexual dimorphism. Oocytes were detected in females from 4–5 mm; this size might be reached by A. muelleri in a few months. The population density increased notably during the summer (June–August). The spawning event date was indirectly estimated from the average size of the oocytes and the percentage of females with free oocytes in the coelom. Both variables were significantly correlated to shallow water temperature (Pearson correlation; P = 0.003 and P = 0.001, respectively). Oocyte size was also significantly correlated to average irradiance level (Pearson correlation; P = 0.044). Spawning took place between August and September (when the water temperature is getting close to its annual maximum) and may last only a few weeks in these temperate waters. The abundance of A. muelleri decreased drastically in September, likely as a consequence of the spawning event effort. Bivalves of the species Epilepton clarkiae were collected together with specimens of A. muelleri living inside the polychaete tubes used as shelters by the sipunculans, with a prevalence of 11.64%. To a lesser extent some Foraminifera were also reported attached to the body of the sipunculans.