Canadian Journal of Forest Research, 1985, 15(2): 410-416.Seedlings representing 10 half-sib families from a black spruce (Piceamariana (Mill.) B.S.P.) stand in central New Brunswick were raised in a greenhouse on three different soils, each with two levels of fertility. Seedling growth varied significantly with soil, fertility, and half-sib family. After 7 months mean dry weights, for example, ranged from 0.292 to 0.755 g with soil, 0.174 to 0.936 g with level of fertility, and 0.370 to 0.657 g with family. With soil and fertility considered fixed variables, the family and family–fertility interaction accounted for approximately 3 and 20% of the phenotypic variation in mean seedling dry weight, respectively. Similar variance components were observed for shoot and root weights, and root collar diameter. Based on the family – soil fertility interaction, the population was divided into groups of (i) broadly adapted families showing average performance through the full range of fertility environments, (ii) strong responders, i.e., families performing below average at low fertility, but showing a large fertilization response, and (iii) weak or nonresponders, i.e., families performing poorly in all fertility environments.