Age, based on analysis of dentinal growth layers, was determined in a sample of 121 harbour porpoises, Phocoena phocoena (L.), from western North Atlantic waters. One growth layer, consisting of a thick opaque zone and a relatively thin translucent zone, is deposited each year.Mean thicknesses of opaque and translucent zones in males and females were 347 μm, 114 μm, 432 μm, and 125 μm, respectively. Significant reduction in thicknesses of growth layers with age was found in both sexes, the major contribution in both cases being progressive reduction in thickness of the opaque zones. Translucent-zone thickness decreased with age in males, but significantly increased in thickness in females. Formation of the opaque zone occurs from June through February, and formation of the translucent zone from January to early September. This overlap is attributed to the protracted calving season of this population, and precludes any simple relationship between food supply and zonation, as proposed by others. Age–length relationships based on numbers of dentinal layers were calculated for males and females using regression analysis. Best fits of body length (b) against age (expressed by completed dentinal layers) (d) were obtained from the curvilinear equations: d = [b/(−1.30b + 209.35)] −1 for males, and d = [b/(−0.84b + 156.15)] −1 for females.