The age and growth of bigeye tuna (Thunnus obesus) from the Western Indian Ocean were investigated using otoliths and first dorsal spines. Microincrements, assumed as daily deposits, were observed on transverse sections of sagittal otoliths from 164 bigeye tuna. The increments were counted to determine age and establish a growth curve. A scanning electron microscope (SEM) was used to count microincrements on fish larger than 100 cm fork length (FL). The von Bertalanffy growth curve is FL = 168.99 (1 - e-0.000879 (t + 123.38)), where FL in cm and t in days were used. The fish reaches 59 cm at 1 year, 111 cm at 3 years and 147 cm at 6 years. The use of SEM is required for counting microincrements on otoliths of bigeye tuna over 120 cm. Growth was also studied by analysing marks on the first dorsal spine of 140 fish. The results obtained with spines and otoliths are comparable until 3 years old, but spines are not suitable for larger fish.