Growth, phenology and frost tolerance of seedlings from 50 populations of Douglas-fir (Pseudotsuga menziesii var. glauca) were compared in 12 environments. Statistical analyses of six variables (bud burst, bud set, 3-year height, spring and fall frost injuries, and deviation from regression of 3-year height on 2-year height) showed that populations not only differed in mean performance, but also reacted differently to the environmental gradient. Most of the population environment interaction was attributable to heterogeneous regressions of population means on environmental means. For all variables except growth rate, the variance of heterogeneous regression coefficients was explained by convergence of regression lines to a common point on the environmental gradient. Consequently, mean values for populations were significantly correlated with regression coefficients. Thus, main effects of populations in those single environments that induced the greatest mean differences reflected the interaction.