Fish size and proximate composition were measured during the fattening process of two groups of bluefin tuna (Thunnus thynnus) kept in ocean cages in the southwestern Mediterranean Sea. The fish were first sampled just after capture in June 1998, and twice more over a month period at the two tuna farms located near Cabo Tiñoso in Murcia, Spain. Both groups were caught within 200 km of each other by purse seining during June. The results found that the mean of tuna fork lengths were significantly different among the three periods sampled, but not so between farms. Additionally, weight is significantly different over time. However, fat content was found to be significantly different between farms and increased from June to November at both locations. Variations in fat levels between both locations could be due to multiples reasons, such as different environmental conditions, feeding protocol or fish density.