The superiority of Hybrid Larch (Larix decidua x Larix kaempferi) over pure parental species has been described by numerous authors for several traits of economic importance. But heterosis or hybrid vigour (that is the superiority of hybrid over mid- or best- parents) has not been so far properly demonstrated because of missing appropriate mating designs including both intra- and inter-specific crossings from the same parents. Taking advantage of a full diallel mating design including both pure and hybrid progenies, we give in this study first results (at 6 years, from 3 locations) on species and family heterosis for adaptive, growth and stem form traits. While positive heterosis is found for most traits, it is mostly negative for stem straightness. But for all traits including stem form, the range of family heterosis is very wide. It is shown that the level of heterosis is highly dependent on the traits considered but also on environmental site conditions.