The sea urchin Paracentrotus lividus was grown on commercial mussel long-lines with the blue mussel, Mytilus edulis, at a farm in Loch Beag, on the west coast of Scotland, to investigate if enhanced sea urchin survivorship and performance (somatic and gonadal) resulted from sea urchin-mussel co-culture system. The sea urchins were fed four diets including: two class sizes of Medulis, (1) large mussels (31.16 ± 5.25 mm shell length), (2) small mussels (18.47 ± 4.86 mm shell length), (3) the kelp Laminaria spp. and (4) no additional feed. The experimental period lasted for 12 mo. No significant difference was observed in survivorship among treatments, ranging from 95.8% to 100%. Final test diameter, linear growth rates (LGR) and specific growth rates were significantly greater for the sea urchins fed the kelp diet compared with sea urchins fed either of the mussel diets and given no additional feed. No significant difference in growth rates was seen between the sea urchins fed the two size classes of mussel. A seasonal variation in growth rates was observed for all the treatment groups with a greater LGR in September to November 2005 compared with January to March 2006. Sea urchins fed on kelp showed significantly greater gonadal growth than the other treatment groups and no significant difference was observed between the sea urchins fed the two size classes of mussel. Gonad coloration in the sea urchins grown on the kelp diet was acceptable or excellent. Minimal roe material in the other treatments prevented color assessment. The results show that P. lividus exhibits high survivorship and linear growth rates when grown on long-lines used for commercial mussel cultivation, even at this northerly latitude. The results suggest that Laminaria spp. is a superior food source for P. lividus compared with the mussel M. edulis, however, a preharvest diet would have to be used to increase roe content prior to harvest even when fed a diet of Laminaria spp. The co-culture of sea urchins and mussels could potentially be implemented globally, wherever rope-grown mussel culture is practiced and sea urchin hatcheries are present. This would enable mussel farms to diversify in the production of a second commercially valuable product, with minimal requirement for new equipment or infrastructure, and would reduce the pressure on the already depleted wild stocks of sea urchins.