Macroalgal detritus has been widely reported as an important source of nutrition in many species of marine bivalves, especially in temperate regions. Consequently, it could be expected that the commercial culture of the mussel Perna canaliculus may benefit from co-culturing with the common kelp Ecklonia radiata in New Zealand. To test this hypothesis a 3-month mesocosm experiment was conducted with kelp alone, mussels alone, or kelp and mussels together in 70 litre mesocosms. All mesocosms received 2.0-3.0 mgC of phytoplankton daily, just above the minimum carbon requirement for mussel growth and water in the mesocosms were refreshed completely every 2 days. Two batches of kelp collected from the same location were used over the course of the experiment and switched out at day 45. Mussel shell length was taken at day 0, 47, and 90. The increase in shell length from day 0-47 were greater for the tanks with mussels alone versus mussels and kelp, and the reverse was found from days 47-90. There was insufficient data of the kelp growth during the first batch due to a die off to make confirm any differences between treatments. Measurements of the second batch of kelp indicated that there was no difference in the kelp growth for kelp alone or with mussels. The results of the study indicate that the quality of the kelp detritus may influence the amount of kelp detritus ingested by the mussels. There was no evidence that the kelp would benefit from nutrients released by the mussels.