Recent studies have suggested that bone alkaline phosphatase (ALP) may be a valuable indicator of skeletal development in wild birds. However, the information about age-related dynamics of ALP isoforms in passerines is very scanty. We examined age-related changes in the activity of bone ALP and liver ALP and tested the applicability of these isoenzymes as indicators of chick maturity in randomly selected nestlings of a small passerine bird, the Great Tit (Parus major). Bone ALP activity was elevated in the middle of the nestling period (day 8), when skeletal growth is assumed to be most rapid, and declined significantly during the prefledging stage (day 15 posthatch). Bone ALP activity at this age was positively and highly significantly related to the overall duration of nestling period and negatively and less significantly related to wing length and body mass. All three morphological traits of the 15-day-old nestlings were nearly significantly negatively correlated with the duration of the nestling period. Liver ALP activity neither changed with nestling age nor was related to nestling morphology. We suggest that prefledging activity of bone ALP is a more reliable indicator of nestling maturity than traditionally used morphological measurements.