Plant structure was shown to limit the seed yield of Lupinus mutabilis through low dry matter production. The genetic and environmental variations of architectural and phenological traits of indeterminate Andean lupin were evaluated. Twelve genotypes were sown at 6–8 dates at 5 locations in Europe in 1994 and 1995. Wide variation was observed for phenological and architectural characters. The variation in the number of mainstem leaves was partly explained by the temperature. The mainstem height and flowering date were related to the number of mainstem leaves but flowering time was also affected independently by the enviromental conditions. The mainstem structure determined the potential number of first-order branches through the number of axillary buds. The actual number of branches was also determined by the prevailing environmental conditions during branch growth. The number of leaves on the second first-order branch was less susceptible to the environmental conditions than the number of leaves on the mainstem. Heritabilities were high for all characters except for the number of leaves on the second first-order branch, and the interactions between genotype and environmental condition were low. Breeding for architecture modification would thus be possible in L. mutabilis .