Age structure of a breeding population of Rana sylvatica from southwestern Quebec, Canada, is described based on the enumeration and analysis of Lines of Arrested Growth (LAGs) in phalanges. Analyzed using skeletochronology ages ranged from 1-4 yr among 98 males and from 2-4 yr among 33 females. Females were on average slightly older than males, but the difference was not significant. Mean snout-vent length was 43.6 mm ± 2.0 (SD) among 179 males and 48.8 mm ± 2.7 among 33 females; the difference between the sexes was significant. Endosteal resorption completely destroyed LAG 1 in 6.1% of frogs, about twice as frequently in males as in females. A minimum of three LAG enumeration readings by two observers differed for 24.4% of preparations and independent readings by the same observer differed for 15.3% of preparations, highlighting the importance of reducing subjectivity in skeletochronological analyses. This was accomplished, in part, by plotting LAG diameters on a histogram whereby outliers from an expected normal distribution indicated loss of inner LAGs via endosteal resorption. Consistent with predictions regarding environmental influences on anuran populations, southern Quebec R. sylvatica matured later and were larger than more southerly conspecifics from a low-elevation area, but matured earlier and were smaller than southerly frogs from a high-elevation area.