Since commercial seaweed cultivation in Tanzania started in 1989, only limited research has been done on impact of stocking density and nutrients, which are factors that may limit seaweed growth. In this study, Eucheuma denticulatum and Kappaphycus alvarezii were cultivated at low (50 g cuttings of seaweed (half of what farmers use) in 4m long nylon ropes) and high (100 g cuttings of seaweed (the stocking density of farmers) in 4m long nylon ropes) stocking densities under in situ growth conditions and the seaweeds were fertilised by nitrogen and phosphorous enrichments. Specific growth rates (SGRs) of both species were significantly higher at the lower stocking density (P<0.05), whether the seaweeds had been fertilised or not. The addition of nutrients had no significant effect (P>0.05) on the seaweeds as the SGRs were similar for both species and densities except for K. alvarezii at high density (P=0.04). Thus, lower stocking density enhances the growth rates of these two seaweeds while additional higher nutrients have little effect.