In 47 families of New Zealand chinook salmon, Oncorhynchus tshawytscha derived from two populations, differing in mean spawning date by 11–17 days and in juvenile life history, neither mean ovum weight nor mean time to hatch differed between the two populations, but substantial differences in mean family weight were apparent from 6 months after fertilization. Differences in growth rates from 12 to 24 months were relatively small, suggesting that most of the divergence in size and growth occurred during the first 6 months of life. There were differences in mean weight for families spawned 11–17 days apart, the approximate interval by which peak spawning date differs between the two populations, but these did not persist beyond 7 months. Differences between the two populations are consistent with their natural life history. The slower growing population experiences cooler temperatures during stream residence and is dominated by fish which spend a year in fresh water before seaward migration, whereas the faster growing population normally experiences warmer temperatures and migrates to sea in the first year of life. These results provide further evidence of life history evolution in partially isolated chinook salmon populations within 90 years (c. 30 generations) of becoming established.