The first dorsal fin spines were used to estimate the age of juvenile bluefin tuna, Thunnus thynnus, from the Mediterranean Sea. Observations on the type of growth zones occurring on the outside margin of each spine section by month and marginal increment analysis indicated that only one translucent zone is formed per year between February and April, when lower sea surface temperatures are observed. Age estimations were based on counts of translucent zones on the spine sections and mean size at estimated age was calculated for the first three years of life.