Brill (Colistium guntheri) and turbot (C. nudipinnis) were aged by counting opaque growth zones in whole and sectioned otoliths. Zones counts from whole otoliths under-estimated age compared with counts from thin otolith sections. Other species of flatfish that have previously been aged from whole otoliths should be re-examined for evidence of age under-estimation, which may be common in species with thick otoliths. Marginal analysis of thin sections supported the hypothesis that one translucent and one opaque zone are formed each year in brill aged 5–10-years old. Marginal analyses for brill greater than 10 years of age, and for turbot, were inconclusive. However, 2+ and 3+ captive-reared turbot deposited the expected number of opaque zones from hatching. Both species grow rapidly for the first three years of life before growth slows appreciably. Turbot grow faster and larger than brill, and females grow faster and larger than males in both species. Growth is minimal in fish older than five years. Maximum observed ages were 21 years for brill and 16 years for turbot.