AbstractMorphological characters, dry matter production and the distribution of14C‐assimilates of the flag leaf were investigated on two spelt varieties (OstroandRouquin) and two wheat varieties (ArinaandProbus) under controlled conditions. Single plants were grown at three different temperature regimes (18/13 °C continuously = T1; 18/13 °C until anthesis and 13/8 °C after anthesis = T2; 13/8 °C until anthesis and 18/13 °C after anthesis = T3).Spelt produced up to 10 tillers more per plant than wheat. However, tiller reduction was also higher for spelt so that both species finally had a similar number of ears per plant.Under favourable conditions (T1) the large single grain weight of spelt, did not fully compensate for its lower kernel number. As a result, the grain yield per ear was 12 % higher in wheat than in spelt. Under low temperatures during grain filling (T2), wheat did not fill it's kernels completely; thus, under these conditions grain yield of spelt was 12 % higher. It seems that the kernel growth of spelt is less limited as compared to wheat (maximum single grain weight reported at T2:Ostro67 mg;Rouquin63 mg;Arina53 mg;Probus47 mg [SE 1.8]).The distribution of thel4C from the flag leaf was similar for both spelt and wheat. At anthesis the highest amount of14C was found in the last two internodes. Three weeks after anthesis, the ear of the main culm was the most important sink (exception T2). Between 48 h after the labelling with14CO2and maturity, different amounts of assimilates were translocated from the stem into the ear of the main shoot and partially also into the tillers.In conclusion significant differences were found between spelt and wheat in tiller formation, grain number per ear, single grain weight and grain yield. Especially under low temperature during grain filling kernel growth of spelt was less limited than that of