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Age-structure and longevity in the euryhaline Common goby, Pomatoschistus microps (Krøyer), has been studied by monthly sampling of estuarine and shore populations in the Isle of Man and in collections from other parts of the British Isles. Age of individuals was determined by examination of caudal peduncle scales, where time of annulus formation was confirmed from monthly changes in mean terminal sclerite width and in frequency of scale-types. After the first winter of life, an annulus of narrow sclerites is produced on resumption of growth in spring and a separate summer annulus, corresponding with a growth retardation during the peak breeding season, is laid down usually in June. Maximum age did not exceed 21–26 months, with most adult fishes dying in the second autumn of life a few months after their first breeding season (within possible age-limits of 12–20 months). Seasonal migration is discussed with reference to the downstream movement of estuarine P. microps when sexually mature after the first winter of life, the winter distribution relative to temperature, and the preference of newly demersal young for less saline parts of the estuary. Cases of mortality from predation and environmental extremes are listed. The question of senescence in relation to life-history pattern and survival in P. microps and some other teleosts is discussed with special reference to gonad maturation and spawning, and the attainment of limiting size.