There is growing demand for the territorial tube-building genus Marphysa (Eunicidae: Polychaete), commonly known in Portugal as “goose”, for use as fishing bait, and it is being harvested all around the world for that purpose. Effects of intraspecific density on juvenile growth were studied over a four-month period in laboratory facilities. Three polychaete densities (low, 50 worms; medium, 150 worms; and high, 250 worms) were used in a 0.25 m² aquarium containing sandy sediment and recirculating water. Total length, dry weight and number of segments were recorded for 60% of the initial population. All polychaetes were also counted to determine mortality rate and territorial behaviour through the existence of body lesions and broken and regenerating posterior segments. The results obtained in this study showed that density had a significant effect (p<0.001) on growth rates in any of the densities studied and that growth was significantly higher at lower densities (p<0.001). For all density levels, estimated daily growth was higher in the first month, decreasing progressively over time. The high aggressiveness and territorial behaviour of Marphysa “sp.” juveniles, well evidenced by the highest mortality (35%) under high density and by the presence of worms with lesions under low density (30%) observed in the first month, suggests that territoriality is probably the main factor involved in the organization and spatial arrangements of individuals within a population. Marphysa juveniles probably compete for burrow space. The results reveal that Marphysa juveniles have a very territorial and aggressive behaviour that should be considered if the species is used for aquaculture production. Additional studies are required to determine the density effects for different developmental stages.