Pinus pseudostrobus Lindl. is a widely distributed species in Mexico that also occurs in Guatemala, Honduras and El Salvador and has been tested outside its natural range in Brazil, Chile, Colombia, New Zealand, South Africa and Venezuela. Due to its great potential, it is necessary to select genotypes capable of increasing its production in the shortest possible time through genetic improvement strategies, where individuals are subjected to conditions forcing them to express their growth potential in advance. The objectives of the present study were to evaluate the genetic variation among half-sib progenies and to estimate heritability (h 2 ) of stem height (SH) and diameter (SD) for seedlings grown under different competitive conditions in a common garden trial. A split-plot experimental design with four replications and three competitive environments (treatments) was used: (I) low inter-family competition (0.25 × 0.12 m), (II) high inter-family competition (0.12 × 0.06 m), and (III) high intra-family competition; 13 half-sib families were assessed, carrying out monthly evaluations for 10 months. Estimated h 2 at individual and family-means levels for both SH and SD varied among competitive environments. For conditions I and II, a trend towards increasing h 2 with age of seedlings was shown, but for condition III, a reverse trend was observed (values close to zero). High genetic stability in SH performance was found in both I and II, and II and III, pairs of environments, as measured by the type-B genetic correlation (0.70 ≤ r B ≤ 1.0), but it tended to decrease after 9 months, when competition increased; genetic stability in SD was lower (0.20 ≤ r B ≤ 0.80) in both pairs of the environments that were sampled. It was concluded that the competition conditions used in field trials for genotype evaluation may significantly affect the variance components, estimation of genetic parameters and genotype stability.